helpers.js

// @flow

import template from "@babel/template";

const helpers = Object.create(null);
export default helpers;

const helper = (minVersion: string) => tpl => ({
  minVersion,
  ast: () => template.program.ast(tpl),
});

helpers.typeof = helper("7.0.0-beta.0")`
  export default function _typeof(obj) {
    if (typeof Symbol === "function" && typeof Symbol.iterator === "symbol") {
      _typeof = function (obj) { return typeof obj; };
    } else {
      _typeof = function (obj) {
        return obj && typeof Symbol === "function" && obj.constructor === Symbol && obj !== Symbol.prototype
          ? "symbol"
          : typeof obj;
      };
    }

    return _typeof(obj);
  }
`;

helpers.jsx = helper("7.0.0-beta.0")`
  var REACT_ELEMENT_TYPE;

  export default function _createRawReactElement(type, props, key, children) {
    if (!REACT_ELEMENT_TYPE) {
      REACT_ELEMENT_TYPE = (
        typeof Symbol === "function" && Symbol.for && Symbol.for("react.element")
      ) || 0xeac7;
    }

    var defaultProps = type && type.defaultProps;
    var childrenLength = arguments.length - 3;

    if (!props && childrenLength !== 0) {
      // If we're going to assign props.children, we create a new object now
      // to avoid mutating defaultProps.
      props = {
        children: void 0,
      };
    }
    if (props && defaultProps) {
      for (var propName in defaultProps) {
        if (props[propName] === void 0) {
          props[propName] = defaultProps[propName];
        }
      }
    } else if (!props) {
      props = defaultProps || {};
    }

    if (childrenLength === 1) {
      props.children = children;
    } else if (childrenLength > 1) {
      var childArray = new Array(childrenLength);
      for (var i = 0; i < childrenLength; i++) {
        childArray[i] = arguments[i + 3];
      }
      props.children = childArray;
    }

    return {
      $$typeof: REACT_ELEMENT_TYPE,
      type: type,
      key: key === undefined ? null : '' + key,
      ref: null,
      props: props,
      _owner: null,
    };
  }
`;

helpers.asyncIterator = helper("7.0.0-beta.0")`
  export default function _asyncIterator(iterable) {
    var method
    if (typeof Symbol === "function") {
      if (Symbol.asyncIterator) {
        method = iterable[Symbol.asyncIterator]
        if (method != null) return method.call(iterable);
      }
      if (Symbol.iterator) {
        method = iterable[Symbol.iterator]
        if (method != null) return method.call(iterable);
      }
    }
    throw new TypeError("Object is not async iterable");
  }
`;

helpers.AwaitValue = helper("7.0.0-beta.0")`
  export default function _AwaitValue(value) {
    this.wrapped = value;
  }
`;

helpers.AsyncGenerator = helper("7.0.0-beta.0")`
  import AwaitValue from "AwaitValue";

  export default function AsyncGenerator(gen) {
    var front, back;

    function send(key, arg) {
      return new Promise(function (resolve, reject) {
        var request = {
          key: key,
          arg: arg,
          resolve: resolve,
          reject: reject,
          next: null,
        };

        if (back) {
          back = back.next = request;
        } else {
          front = back = request;
          resume(key, arg);
        }
      });
    }

    function resume(key, arg) {
      try {
        var result = gen[key](arg)
        var value = result.value;
        var wrappedAwait = value instanceof AwaitValue;

        Promise.resolve(wrappedAwait ? value.wrapped : value).then(
          function (arg) {
            if (wrappedAwait) {
              resume("next", arg);
              return
            }

            settle(result.done ? "return" : "normal", arg);
          },
          function (err) { resume("throw", err); });
      } catch (err) {
        settle("throw", err);
      }
    }

    function settle(type, value) {
      switch (type) {
        case "return":
          front.resolve({ value: value, done: true });
          break;
        case "throw":
          front.reject(value);
          break;
        default:
          front.resolve({ value: value, done: false });
          break;
      }

      front = front.next;
      if (front) {
        resume(front.key, front.arg);
      } else {
        back = null;
      }
    }

    this._invoke = send;

    // Hide "return" method if generator return is not supported
    if (typeof gen.return !== "function") {
      this.return = undefined;
    }
  }

  if (typeof Symbol === "function" && Symbol.asyncIterator) {
    AsyncGenerator.prototype[Symbol.asyncIterator] = function () { return this; };
  }

  AsyncGenerator.prototype.next = function (arg) { return this._invoke("next", arg); };
  AsyncGenerator.prototype.throw = function (arg) { return this._invoke("throw", arg); };
  AsyncGenerator.prototype.return = function (arg) { return this._invoke("return", arg); };
`;

helpers.wrapAsyncGenerator = helper("7.0.0-beta.0")`
  import AsyncGenerator from "AsyncGenerator";

  export default function _wrapAsyncGenerator(fn) {
    return function () {
      return new AsyncGenerator(fn.apply(this, arguments));
    };
  }
`;

helpers.awaitAsyncGenerator = helper("7.0.0-beta.0")`
  import AwaitValue from "AwaitValue";

  export default function _awaitAsyncGenerator(value) {
    return new AwaitValue(value);
  }
`;

helpers.asyncGeneratorDelegate = helper("7.0.0-beta.0")`
  export default function _asyncGeneratorDelegate(inner, awaitWrap) {
    var iter = {}, waiting = false;

    function pump(key, value) {
      waiting = true;
      value = new Promise(function (resolve) { resolve(inner[key](value)); });
      return { done: false, value: awaitWrap(value) };
    };

    if (typeof Symbol === "function" && Symbol.iterator) {
      iter[Symbol.iterator] = function () { return this; };
    }

    iter.next = function (value) {
      if (waiting) {
        waiting = false;
        return value;
      }
      return pump("next", value);
    };

    if (typeof inner.throw === "function") {
      iter.throw = function (value) {
        if (waiting) {
          waiting = false;
          throw value;
        }
        return pump("throw", value);
      };
    }

    if (typeof inner.return === "function") {
      iter.return = function (value) {
        return pump("return", value);
      };
    }

    return iter;
  }
`;

helpers.asyncToGenerator = helper("7.0.0-beta.0")`
  function asyncGeneratorStep(gen, resolve, reject, _next, _throw, key, arg) {
    try {
      var info = gen[key](arg);
      var value = info.value;
    } catch (error) {
      reject(error);
      return;
    }

    if (info.done) {
      resolve(value);
    } else {
      Promise.resolve(value).then(_next, _throw);
    }
  }

  export default function _asyncToGenerator(fn) {
    return function () {
      var self = this, args = arguments;
      return new Promise(function (resolve, reject) {
        var gen = fn.apply(self, args);
        function _next(value) {
          asyncGeneratorStep(gen, resolve, reject, _next, _throw, "next", value);
        }
        function _throw(err) {
          asyncGeneratorStep(gen, resolve, reject, _next, _throw, "throw", err);
        }

        _next(undefined);
      });
    };
  }
`;

helpers.classCallCheck = helper("7.0.0-beta.0")`
  export default function _classCallCheck(instance, Constructor) {
    if (!(instance instanceof Constructor)) {
      throw new TypeError("Cannot call a class as a function");
    }
  }
`;

helpers.createClass = helper("7.0.0-beta.0")`
  function _defineProperties(target, props) {
    for (var i = 0; i < props.length; i ++) {
      var descriptor = props[i];
      descriptor.enumerable = descriptor.enumerable || false;
      descriptor.configurable = true;
      if ("value" in descriptor) descriptor.writable = true;
      Object.defineProperty(target, descriptor.key, descriptor);
    }
  }

  export default function _createClass(Constructor, protoProps, staticProps) {
    if (protoProps) _defineProperties(Constructor.prototype, protoProps);
    if (staticProps) _defineProperties(Constructor, staticProps);
    return Constructor;
  }
`;

helpers.defineEnumerableProperties = helper("7.0.0-beta.0")`
  export default function _defineEnumerableProperties(obj, descs) {
    for (var key in descs) {
      var desc = descs[key];
      desc.configurable = desc.enumerable = true;
      if ("value" in desc) desc.writable = true;
      Object.defineProperty(obj, key, desc);
    }

    // Symbols are not enumerated over by for-in loops. If native
    // Symbols are available, fetch all of the descs object's own
    // symbol properties and define them on our target object too.
    if (Object.getOwnPropertySymbols) {
      var objectSymbols = Object.getOwnPropertySymbols(descs);
      for (var i = 0; i < objectSymbols.length; i++) {
        var sym = objectSymbols[i];
        var desc = descs[sym];
        desc.configurable = desc.enumerable = true;
        if ("value" in desc) desc.writable = true;
        Object.defineProperty(obj, sym, desc);
      }
    }
    return obj;
  }
`;

helpers.defaults = helper("7.0.0-beta.0")`
  export default function _defaults(obj, defaults) {
    var keys = Object.getOwnPropertyNames(defaults);
    for (var i = 0; i < keys.length; i++) {
      var key = keys[i];
      var value = Object.getOwnPropertyDescriptor(defaults, key);
      if (value && value.configurable && obj[key] === undefined) {
        Object.defineProperty(obj, key, value);
      }
    }
    return obj;
  }
`;

helpers.defineProperty = helper("7.0.0-beta.0")`
  export default function _defineProperty(obj, key, value) {
    // Shortcircuit the slow defineProperty path when possible.
    // We are trying to avoid issues where setters defined on the
    // prototype cause side effects under the fast path of simple
    // assignment. By checking for existence of the property with
    // the in operator, we can optimize most of this overhead away.
    if (key in obj) {
      Object.defineProperty(obj, key, {
        value: value,
        enumerable: true,
        configurable: true,
        writable: true
      });
    } else {
      obj[key] = value;
    }
    return obj;
  }
`;

helpers.extends = helper("7.0.0-beta.0")`
  export default function _extends() {
    _extends = Object.assign || function (target) {
      for (var i = 1; i < arguments.length; i++) {
        var source = arguments[i];
        for (var key in source) {
          if (Object.prototype.hasOwnProperty.call(source, key)) {
            target[key] = source[key];
          }
        }
      }
      return target;
    };

    return _extends.apply(this, arguments);
  }
`;

helpers.objectSpread = helper("7.0.0-beta.0")`
  import defineProperty from "defineProperty";

  export default function _objectSpread(target) {
    for (var i = 1; i < arguments.length; i++) {
      if (i % 2) {
        var source = (arguments[i] != null) ? arguments[i] : {};
        var ownKeys = Object.keys(source);
        if (typeof Object.getOwnPropertySymbols === 'function') {
          ownKeys = ownKeys.concat(Object.getOwnPropertySymbols(source).filter(function(sym) {
            return Object.getOwnPropertyDescriptor(source, sym).enumerable;
          }));
        }
        ownKeys.forEach(function(key) {
          defineProperty(target, key, source[key]);
        });
      } else {
        Object.defineProperties(target, Object.getOwnPropertyDescriptors(arguments[i]));
      }
    }
    return target;
  }
`;

helpers.inherits = helper("7.0.0-beta.0")`
  import setPrototypeOf from "setPrototypeOf";

  export default function _inherits(subClass, superClass) {
    if (typeof superClass !== "function" && superClass !== null) {
      throw new TypeError("Super expression must either be null or a function");
    }
    subClass.prototype = Object.create(superClass && superClass.prototype, {
      constructor: {
        value: subClass,
        writable: true,
        configurable: true
      }
    });
    if (superClass) setPrototypeOf(subClass, superClass);
  }
`;

helpers.inheritsLoose = helper("7.0.0-beta.0")`
  export default function _inheritsLoose(subClass, superClass) {
    subClass.prototype = Object.create(superClass.prototype);
    subClass.prototype.constructor = subClass;
    subClass.__proto__ = superClass;
  }
`;

helpers.getPrototypeOf = helper("7.0.0-beta.0")`
  export default function _getPrototypeOf(o) {
    _getPrototypeOf = Object.setPrototypeOf
      ? Object.getPrototypeOf
      : function _getPrototypeOf(o) {
          return o.__proto__ || Object.getPrototypeOf(o);
        };
    return _getPrototypeOf(o);
  }
`;

helpers.setPrototypeOf = helper("7.0.0-beta.0")`
  export default function _setPrototypeOf(o, p) {
    _setPrototypeOf = Object.setPrototypeOf || function _setPrototypeOf(o, p) {
      o.__proto__ = p;
      return o;
    };
    return _setPrototypeOf(o, p);
  }
`;

helpers.construct = helper("7.0.0-beta.0")`
  import setPrototypeOf from "setPrototypeOf";

  function isNativeReflectConstruct() {
    if (typeof Reflect === "undefined" || !Reflect.construct) return false;

    // core-js@3
    if (Reflect.construct.sham) return false;

    // Proxy can't be polyfilled. Every browser implemented
    // proxies before or at the same time as Reflect.construct,
    // so if they support Proxy they also support Reflect.construct.
    if (typeof Proxy === "function") return true;

    // Since Reflect.construct can't be properly polyfilled, some
    // implementations (e.g. core-js@2) don't set the correct internal slots.
    // Those polyfills don't allow us to subclass built-ins, so we need to
    // use our fallback implementation.
    try {
      // If the internal slots aren't set, this throws an error similar to
      //   TypeError: this is not a Date object.
      Date.prototype.toString.call(Reflect.construct(Date, [], function() {}));
      return true;
    } catch (e) {
      return false;
    }
  }

  export default function _construct(Parent, args, Class) {
    if (isNativeReflectConstruct()) {
      _construct = Reflect.construct;
    } else {
      // NOTE: If Parent !== Class, the correct __proto__ is set *after*
      //       calling the constructor.
      _construct = function _construct(Parent, args, Class) {
        var a = [null];
        a.push.apply(a, args);
        var Constructor = Function.bind.apply(Parent, a);
        var instance = new Constructor();
        if (Class) setPrototypeOf(instance, Class.prototype);
        return instance;
      };
    }
    // Avoid issues with Class being present but undefined when it wasn't
    // present in the original call.
    return _construct.apply(null, arguments);
  }
`;

helpers.isNativeFunction = helper("7.0.0-beta.0")`
  export default function _isNativeFunction(fn) {
    // Note: This function returns "true" for core-js functions.
    return Function.toString.call(fn).indexOf("[native code]") !== -1;
  }
`;

// Based on https://github.com/WebReflection/babel-plugin-transform-builtin-classes
helpers.wrapNativeSuper = helper("7.0.0-beta.0")`
  import getPrototypeOf from "getPrototypeOf";
  import setPrototypeOf from "setPrototypeOf";
  import isNativeFunction from "isNativeFunction";
  import construct from "construct";

  export default function _wrapNativeSuper(Class) {
    var _cache = typeof Map === "function" ? new Map() : undefined;

    _wrapNativeSuper = function _wrapNativeSuper(Class) {
      if (Class === null || !isNativeFunction(Class)) return Class;
      if (typeof Class !== "function") {
        throw new TypeError("Super expression must either be null or a function");
      }
      if (typeof _cache !== "undefined") {
        if (_cache.has(Class)) return _cache.get(Class);
        _cache.set(Class, Wrapper);
      }
      function Wrapper() {
        return construct(Class, arguments, getPrototypeOf(this).constructor)
      }
      Wrapper.prototype = Object.create(Class.prototype, {
        constructor: {
          value: Wrapper,
          enumerable: false,
          writable: true,
          configurable: true,
        }
      });

      return setPrototypeOf(Wrapper, Class);
    }

    return _wrapNativeSuper(Class)
  }
`;

helpers.instanceof = helper("7.0.0-beta.0")`
  export default function _instanceof(left, right) {
    if (right != null && typeof Symbol !== "undefined" && right[Symbol.hasInstance]) {
      return right[Symbol.hasInstance](left);
    } else {
      return left instanceof right;
    }
  }
`;

helpers.interopRequireDefault = helper("7.0.0-beta.0")`
  export default function _interopRequireDefault(obj) {
    return obj && obj.__esModule ? obj : { default: obj };
  }
`;

helpers.interopRequireWildcard = helper("7.0.0-beta.0")`
  export default function _interopRequireWildcard(obj) {
    if (obj && obj.__esModule) {
      return obj;
    } else {
      var newObj = {};
      if (obj != null) {
        for (var key in obj) {
          if (Object.prototype.hasOwnProperty.call(obj, key)) {
            var desc = Object.defineProperty && Object.getOwnPropertyDescriptor
              ? Object.getOwnPropertyDescriptor(obj, key)
              : {};
            if (desc.get || desc.set) {
              Object.defineProperty(newObj, key, desc);
            } else {
              newObj[key] = obj[key];
            }
          }
        }
      }
      newObj.default = obj;
      return newObj;
    }
  }
`;

helpers.newArrowCheck = helper("7.0.0-beta.0")`
  export default function _newArrowCheck(innerThis, boundThis) {
    if (innerThis !== boundThis) {
      throw new TypeError("Cannot instantiate an arrow function");
    }
  }
`;

helpers.objectDestructuringEmpty = helper("7.0.0-beta.0")`
  export default function _objectDestructuringEmpty(obj) {
    if (obj == null) throw new TypeError("Cannot destructure undefined");
  }
`;

helpers.objectWithoutPropertiesLoose = helper("7.0.0-beta.0")`
  export default function _objectWithoutPropertiesLoose(source, excluded) {
    if (source == null) return {};

    var target = {};
    var sourceKeys = Object.keys(source);
    var key, i;

    for (i = 0; i < sourceKeys.length; i++) {
      key = sourceKeys[i];
      if (excluded.indexOf(key) >= 0) continue;
      target[key] = source[key];
    }

    return target;
  }
`;

helpers.objectWithoutProperties = helper("7.0.0-beta.0")`
  import objectWithoutPropertiesLoose from "objectWithoutPropertiesLoose";

  export default function _objectWithoutProperties(source, excluded) {
    if (source == null) return {};

    var target = objectWithoutPropertiesLoose(source, excluded);
    var key, i;

    if (Object.getOwnPropertySymbols) {
      var sourceSymbolKeys = Object.getOwnPropertySymbols(source);
      for (i = 0; i < sourceSymbolKeys.length; i++) {
        key = sourceSymbolKeys[i];
        if (excluded.indexOf(key) >= 0) continue;
        if (!Object.prototype.propertyIsEnumerable.call(source, key)) continue;
        target[key] = source[key];
      }
    }

    return target;
  }
`;

helpers.assertThisInitialized = helper("7.0.0-beta.0")`
  export default function _assertThisInitialized(self) {
    if (self === void 0) {
      throw new ReferenceError("this hasn't been initialised - super() hasn't been called");
    }
    return self;
  }
`;

helpers.possibleConstructorReturn = helper("7.0.0-beta.0")`
  import assertThisInitialized from "assertThisInitialized";

  export default function _possibleConstructorReturn(self, call) {
    if (call && (typeof call === "object" || typeof call === "function")) {
      return call;
    }
    return assertThisInitialized(self);
  }
`;

helpers.superPropBase = helper("7.0.0-beta.0")`
  import getPrototypeOf from "getPrototypeOf";

  export default function _superPropBase(object, property) {
    // Yes, this throws if object is null to being with, that's on purpose.
    while (!Object.prototype.hasOwnProperty.call(object, property)) {
      object = getPrototypeOf(object);
      if (object === null) break;
    }
    return object;
  }
`;

helpers.get = helper("7.0.0-beta.0")`
  import getPrototypeOf from "getPrototypeOf";
  import superPropBase from "superPropBase";

  export default function _get(target, property, receiver) {
    if (typeof Reflect !== "undefined" && Reflect.get) {
      _get = Reflect.get;
    } else {
      _get = function _get(target, property, receiver) {
        var base = superPropBase(target, property);

        if (!base) return;

        var desc = Object.getOwnPropertyDescriptor(base, property);
        if (desc.get) {
          return desc.get.call(receiver);
        }

        return desc.value;
      };
    }
    return _get(target, property, receiver || target);
  }
`;

helpers.set = helper("7.0.0-beta.0")`
  import getPrototypeOf from "getPrototypeOf";
  import superPropBase from "superPropBase";
  import defineProperty from "defineProperty";

  function set(target, property, value, receiver) {
    if (typeof Reflect !== "undefined" && Reflect.set) {
      set = Reflect.set;
    } else {
      set = function set(target, property, value, receiver) {
        var base = superPropBase(target, property);
        var desc;

        if (base) {
          desc = Object.getOwnPropertyDescriptor(base, property);
          if (desc.set) {
            desc.set.call(receiver, value);
            return true;
          } else if (!desc.writable) {
            // Both getter and non-writable fall into this.
            return false;
          }
        }

        // Without a super that defines the property, spec boils down to
        // "define on receiver" for some reason.
        desc = Object.getOwnPropertyDescriptor(receiver, property);
        if (desc) {
          if (!desc.writable) {
            // Setter, getter, and non-writable fall into this.
            return false;
          }

          desc.value = value;
          Object.defineProperty(receiver, property, desc);
        } else {
          // Avoid setters that may be defined on Sub's prototype, but not on
          // the instance.
          defineProperty(receiver, property, value);
        }

        return true;
      };
    }

    return set(target, property, value, receiver);
  }

  export default function _set(target, property, value, receiver, isStrict) {
    var s = set(target, property, value, receiver || target);
    if (!s && isStrict) {
      throw new Error('failed to set property');
    }

    return value;
  }
`;

helpers.taggedTemplateLiteral = helper("7.0.0-beta.0")`
  export default function _taggedTemplateLiteral(strings, raw) {
    if (!raw) { raw = strings.slice(0); }
    return Object.freeze(Object.defineProperties(strings, {
        raw: { value: Object.freeze(raw) }
    }));
  }
`;

helpers.taggedTemplateLiteralLoose = helper("7.0.0-beta.0")`
  export default function _taggedTemplateLiteralLoose(strings, raw) {
    if (!raw) { raw = strings.slice(0); }
    strings.raw = raw;
    return strings;
  }
`;

helpers.temporalRef = helper("7.0.0-beta.0")`
  import undef from "temporalUndefined";

  export default function _temporalRef(val, name) {
    if (val === undef) {
      throw new ReferenceError(name + " is not defined - temporal dead zone");
    } else {
      return val;
    }
  }
`;

helpers.readOnlyError = helper("7.0.0-beta.0")`
  export default function _readOnlyError(name) {
    throw new Error("\\"" + name + "\\" is read-only");
  }
`;

helpers.classNameTDZError = helper("7.0.0-beta.0")`
  export default function _classNameTDZError(name) {
    throw new Error("Class \\"" + name + "\\" cannot be referenced in computed property keys.");
  }
`;

helpers.temporalUndefined = helper("7.0.0-beta.0")`
  export default {};
`;

helpers.slicedToArray = helper("7.0.0-beta.0")`
  import arrayWithHoles from "arrayWithHoles";
  import iterableToArrayLimit from "iterableToArrayLimit";
  import nonIterableRest from "nonIterableRest";

  export default function _slicedToArray(arr, i) {
    return arrayWithHoles(arr) || iterableToArrayLimit(arr, i) || nonIterableRest();
  }
`;

helpers.slicedToArrayLoose = helper("7.0.0-beta.0")`
  import arrayWithHoles from "arrayWithHoles";
  import iterableToArrayLimitLoose from "iterableToArrayLimitLoose";
  import nonIterableRest from "nonIterableRest";

  export default function _slicedToArrayLoose(arr, i) {
    return arrayWithHoles(arr) || iterableToArrayLimitLoose(arr, i) || nonIterableRest();
  }
`;

helpers.toArray = helper("7.0.0-beta.0")`
  import arrayWithHoles from "arrayWithHoles";
  import iterableToArray from "iterableToArray";
  import nonIterableRest from "nonIterableRest";

  export default function _toArray(arr) {
    return arrayWithHoles(arr) || iterableToArray(arr) || nonIterableRest();
  }
`;

helpers.toConsumableArray = helper("7.0.0-beta.0")`
  import arrayWithoutHoles from "arrayWithoutHoles";
  import iterableToArray from "iterableToArray";
  import nonIterableSpread from "nonIterableSpread";

  export default function _toConsumableArray(arr) {
    return arrayWithoutHoles(arr) || iterableToArray(arr) || nonIterableSpread();
  }
`;

helpers.arrayWithoutHoles = helper("7.0.0-beta.0")`
  export default function _arrayWithoutHoles(arr) {
    if (Array.isArray(arr)) {
      for (var i = 0, arr2 = new Array(arr.length); i < arr.length; i++) arr2[i] = arr[i];
      return arr2;
    }
  }
`;

helpers.arrayWithHoles = helper("7.0.0-beta.0")`
  export default function _arrayWithHoles(arr) {
    if (Array.isArray(arr)) return arr;
  }
`;

helpers.iterableToArray = helper("7.0.0-beta.0")`
  export default function _iterableToArray(iter) {
    if (
      Symbol.iterator in Object(iter) ||
      Object.prototype.toString.call(iter) === "[object Arguments]"
    ) return Array.from(iter);
  }
`;

helpers.iterableToArrayLimit = helper("7.0.0-beta.0")`
  export default function _iterableToArrayLimit(arr, i) {
    // this is an expanded form of \`for...of\` that properly supports abrupt completions of
    // iterators etc. variable names have been minimised to reduce the size of this massive
    // helper. sometimes spec compliancy is annoying :(
    //
    // _n = _iteratorNormalCompletion
    // _d = _didIteratorError
    // _e = _iteratorError
    // _i = _iterator
    // _s = _step

    var _arr = [];
    var _n = true;
    var _d = false;
    var _e = undefined;
    try {
      for (var _i = arr[Symbol.iterator](), _s; !(_n = (_s = _i.next()).done); _n = true) {
        _arr.push(_s.value);
        if (i && _arr.length === i) break;
      }
    } catch (err) {
      _d = true;
      _e = err;
    } finally {
      try {
        if (!_n && _i["return"] != null) _i["return"]();
      } finally {
        if (_d) throw _e;
      }
    }
    return _arr;
  }
`;

helpers.iterableToArrayLimitLoose = helper("7.0.0-beta.0")`
  export default function _iterableToArrayLimitLoose(arr, i) {
    var _arr = [];
    for (var _iterator = arr[Symbol.iterator](), _step; !(_step = _iterator.next()).done;) {
      _arr.push(_step.value);
      if (i && _arr.length === i) break;
    }
    return _arr;
  }
`;

helpers.nonIterableSpread = helper("7.0.0-beta.0")`
  export default function _nonIterableSpread() {
    throw new TypeError("Invalid attempt to spread non-iterable instance");
  }
`;

helpers.nonIterableRest = helper("7.0.0-beta.0")`
  export default function _nonIterableRest() {
    throw new TypeError("Invalid attempt to destructure non-iterable instance");
  }
`;

helpers.skipFirstGeneratorNext = helper("7.0.0-beta.0")`
  export default function _skipFirstGeneratorNext(fn) {
    return function () {
      var it = fn.apply(this, arguments);
      it.next();
      return it;
    }
  }
`;

helpers.toPrimitive = helper("7.1.5")`
  export default function _toPrimitive(
    input,
    hint /*: "default" | "string" | "number" | void */
  ) {
    if (typeof input !== "object" || input === null) return input;
    var prim = input[Symbol.toPrimitive];
    if (prim !== undefined) {
      var res = prim.call(input, hint || "default");
      if (typeof res !== "object") return res;
      throw new TypeError("@@toPrimitive must return a primitive value.");
    }
    return (hint === "string" ? String : Number)(input);
  }
`;

helpers.toPropertyKey = helper("7.1.5")`
  import toPrimitive from "toPrimitive";

  export default function _toPropertyKey(arg) {
    var key = toPrimitive(arg, "string");
    return typeof key === "symbol" ? key : String(key);
  }
`;

/**
 * Add a helper that will throw a useful error if the transform fails to detect the class
 * property assignment, so users know something failed.
 */
helpers.initializerWarningHelper = helper("7.0.0-beta.0")`
    export default function _initializerWarningHelper(descriptor, context){
        throw new Error(
          'Decorating class property failed. Please ensure that ' +
          'proposal-class-properties is enabled and set to use loose mode. ' +
          'To use proposal-class-properties in spec mode with decorators, wait for ' +
          'the next major version of decorators in stage 2.'
        );
    }
`;

/**
 * Add a helper to call as a replacement for class property definition.
 */
helpers.initializerDefineProperty = helper("7.0.0-beta.0")`
    export default function _initializerDefineProperty(target, property, descriptor, context){
        if (!descriptor) return;

        Object.defineProperty(target, property, {
            enumerable: descriptor.enumerable,
            configurable: descriptor.configurable,
            writable: descriptor.writable,
            value: descriptor.initializer ? descriptor.initializer.call(context) : void 0,
        });
    }
`;

/**
 * Add a helper to take an initial descriptor, apply some decorators to it, and optionally
 * define the property.
 */
helpers.applyDecoratedDescriptor = helper("7.0.0-beta.0")`
    export default function _applyDecoratedDescriptor(target, property, decorators, descriptor, context){
        var desc = {};
        Object['ke' + 'ys'](descriptor).forEach(function(key){
            desc[key] = descriptor[key];
        });
        desc.enumerable = !!desc.enumerable;
        desc.configurable = !!desc.configurable;
        if ('value' in desc || desc.initializer){
            desc.writable = true;
        }

        desc = decorators.slice().reverse().reduce(function(desc, decorator){
            return decorator(target, property, desc) || desc;
        }, desc);

        if (context && desc.initializer !== void 0){
            desc.value = desc.initializer ? desc.initializer.call(context) : void 0;
            desc.initializer = undefined;
        }

        if (desc.initializer === void 0){
            // This is a hack to avoid this being processed by 'transform-runtime'.
            // See issue #9.
            Object['define' + 'Property'](target, property, desc);
            desc = null;
        }

        return desc;
    }
`;

helpers.classPrivateFieldLooseKey = helper("7.0.0-beta.0")`
  var id = 0;
  export default function _classPrivateFieldKey(name) {
    return "__private_" + (id++) + "_" + name;
  }
`;

helpers.classPrivateFieldLooseBase = helper("7.0.0-beta.0")`
  export default function _classPrivateFieldBase(receiver, privateKey) {
    if (!Object.prototype.hasOwnProperty.call(receiver, privateKey)) {
      throw new TypeError("attempted to use private field on non-instance");
    }
    return receiver;
  }
`;

helpers.classPrivateFieldGet = helper("7.0.0-beta.0")`
  export default function _classPrivateFieldGet(receiver, privateMap) {
    if (!privateMap.has(receiver)) {
      throw new TypeError("attempted to get private field on non-instance");
    }
    var descriptor = privateMap.get(receiver);
    if (descriptor.get) {
      return descriptor.get.call(receiver);
    }
    return descriptor.value;
  }
`;

helpers.classPrivateFieldSet = helper("7.0.0-beta.0")`
  export default function _classPrivateFieldSet(receiver, privateMap, value) {
    if (!privateMap.has(receiver)) {
      throw new TypeError("attempted to set private field on non-instance");
    }
    var descriptor = privateMap.get(receiver);
    if (descriptor.set) {
      descriptor.set.call(receiver, value);
    } else {
      if (!descriptor.writable) {
        // This should only throw in strict mode, but class bodies are
        // always strict and private fields can only be used inside
        // class bodies.
        throw new TypeError("attempted to set read only private field");
      }

      descriptor.value = value;
    }

    return value;
  }
`;

helpers.classStaticPrivateFieldSpecGet = helper("7.0.2")`
  export default function _classStaticPrivateFieldSpecGet(receiver, classConstructor, descriptor) {
    if (receiver !== classConstructor) {
      throw new TypeError("Private static access of wrong provenance");
    }
    return descriptor.value;
  }
`;

helpers.classStaticPrivateFieldSpecSet = helper("7.0.2")`
  export default function _classStaticPrivateFieldSpecSet(receiver, classConstructor, descriptor, value) {
    if (receiver !== classConstructor) {
      throw new TypeError("Private static access of wrong provenance");
    }
    if (!descriptor.writable) {
      // This should only throw in strict mode, but class bodies are
      // always strict and private fields can only be used inside
      // class bodies.
      throw new TypeError("attempted to set read only private field");
    }
    descriptor.value = value;
    return value;
  }
  
`;

helpers.decorate = helper("7.1.5")`
  import toArray from "toArray";
  import toPropertyKey from "toPropertyKey";

  // These comments are stripped by @babel/template
  /*::
  type PropertyDescriptor =
    | {
        value: any,
        writable: boolean,
        configurable: boolean,
        enumerable: boolean,
      }
    | {
        get?: () => any,
        set?: (v: any) => void,
        configurable: boolean,
        enumerable: boolean,
      };

  type FieldDescriptor ={
    writable: boolean,
    configurable: boolean,
    enumerable: boolean,
  };

  type Placement = "static" | "prototype" | "own";
  type Key = string | symbol; // PrivateName is not supported yet.

  type ElementDescriptor =
    | {
        kind: "method",
        key: Key,
        placement: Placement,
        descriptor: PropertyDescriptor
      }
    | {
        kind: "field",
        key: Key,
        placement: Placement,
        descriptor: FieldDescriptor,
        initializer?: () => any,
      };

  // This is exposed to the user code
  type ElementObjectInput = ElementDescriptor & {
    [@@toStringTag]?: "Descriptor"
  };

  // This is exposed to the user code
  type ElementObjectOutput = ElementDescriptor & {
    [@@toStringTag]?: "Descriptor"
    extras?: ElementDescriptor[],
    finisher?: ClassFinisher,
  };

  // This is exposed to the user code
  type ClassObject = {
    [@@toStringTag]?: "Descriptor",
    kind: "class",
    elements: ElementDescriptor[],
  };

  type ElementDecorator = (descriptor: ElementObjectInput) => ?ElementObjectOutput;
  type ClassDecorator = (descriptor: ClassObject) => ?ClassObject;
  type ClassFinisher = <A, B>(cl: Class<A>) => Class<B>;

  // Only used by Babel in the transform output, not part of the spec.
  type ElementDefinition =
    | {
        kind: "method",
        value: any,
        key: Key,
        static?: boolean,
        decorators?: ElementDecorator[],
      }
    | {
        kind: "field",
        value: () => any,
        key: Key,
        static?: boolean,
        decorators?: ElementDecorator[],
    };

  declare function ClassFactory<C>(initialize: (instance: C) => void): {
    F: Class<C>,
    d: ElementDefinition[]
  }

  */

  /*::
  // Various combinations with/without extras and with one or many finishers

  type ElementFinisherExtras = {
    element: ElementDescriptor,
    finisher?: ClassFinisher,
    extras?: ElementDescriptor[],
  };

  type ElementFinishersExtras = {
    element: ElementDescriptor,
    finishers: ClassFinisher[],
    extras: ElementDescriptor[],
  };

  type ElementsFinisher = {
    elements: ElementDescriptor[],
    finisher?: ClassFinisher,
  };

  type ElementsFinishers = {
    elements: ElementDescriptor[],
    finishers: ClassFinisher[],
  };

  */

  /*::

  type Placements = {
    static: Key[],
    prototype: Key[],
    own: Key[],
  };

  */

  // ClassDefinitionEvaluation (Steps 26-*)
  export default function _decorate(
    decorators /*: ClassDecorator[] */,
    factory /*: ClassFactory */,
    superClass /*: ?Class<*> */,
    mixins /*: ?Array<Function> */,
  ) /*: Class<*> */ {
    var api = _getDecoratorsApi();
    if (mixins) {
      for (var i = 0; i < mixins.length; i++) {
        api = mixins[i](api);
      }
    }

    var r = factory(function initialize(O) {
      api.initializeInstanceElements(O, decorated.elements);
    }, superClass);
    var decorated = api.decorateClass(
      _coalesceClassElements(r.d.map(_createElementDescriptor)),
      decorators,
    );

    api.initializeClassElements(r.F, decorated.elements);

    return api.runClassFinishers(r.F, decorated.finishers);
  }

  function _getDecoratorsApi() {
    _getDecoratorsApi = function() {
      return api;
    };

    var api = {
      elementsDefinitionOrder: [["method"], ["field"]],

      // InitializeInstanceElements
      initializeInstanceElements: function(
        /*::<C>*/ O /*: C */,
        elements /*: ElementDescriptor[] */,
      ) {
        ["method", "field"].forEach(function(kind) {
          elements.forEach(function(element /*: ElementDescriptor */) {
            if (element.kind === kind && element.placement === "own") {
              this.defineClassElement(O, element);
            }
          }, this);
        }, this);
      },

      // InitializeClassElements
      initializeClassElements: function(
        /*::<C>*/ F /*: Class<C> */,
        elements /*: ElementDescriptor[] */,
      ) {
        var proto = F.prototype;

        ["method", "field"].forEach(function(kind) {
          elements.forEach(function(element /*: ElementDescriptor */) {
            var placement = element.placement;
            if (
              element.kind === kind &&
              (placement === "static" || placement === "prototype")
            ) {
              var receiver = placement === "static" ? F : proto;
              this.defineClassElement(receiver, element);
            }
          }, this);
        }, this);
      },

      // DefineClassElement
      defineClassElement: function(
        /*::<C>*/ receiver /*: C | Class<C> */,
        element /*: ElementDescriptor */,
      ) {
        var descriptor /*: PropertyDescriptor */ = element.descriptor;
        if (element.kind === "field") {
          var initializer = element.initializer;
          descriptor = {
            enumerable: descriptor.enumerable,
            writable: descriptor.writable,
            configurable: descriptor.configurable,
            value: initializer === void 0 ? void 0 : initializer.call(receiver),
          };
        }
        Object.defineProperty(receiver, element.key, descriptor);
      },

      // DecorateClass
      decorateClass: function(
        elements /*: ElementDescriptor[] */,
        decorators /*: ClassDecorator[] */,
      ) /*: ElementsFinishers */ {
        var newElements /*: ElementDescriptor[] */ = [];
        var finishers /*: ClassFinisher[] */ = [];
        var placements /*: Placements */ = {
          static: [],
          prototype: [],
          own: [],
        };

        elements.forEach(function(element /*: ElementDescriptor */) {
          this.addElementPlacement(element, placements);
        }, this);

        elements.forEach(function(element /*: ElementDescriptor */) {
          if (!_hasDecorators(element)) return newElements.push(element);

          var elementFinishersExtras /*: ElementFinishersExtras */ = this.decorateElement(
            element,
            placements,
          );
          newElements.push(elementFinishersExtras.element);
          newElements.push.apply(newElements, elementFinishersExtras.extras);
          finishers.push.apply(finishers, elementFinishersExtras.finishers);
        }, this);

        if (!decorators) {
          return { elements: newElements, finishers: finishers };
        }

        var result /*: ElementsFinishers */ = this.decorateConstructor(
          newElements,
          decorators,
        );
        finishers.push.apply(finishers, result.finishers);
        result.finishers = finishers;

        return result;
      },

      // AddElementPlacement
      addElementPlacement: function(
        element /*: ElementDescriptor */,
        placements /*: Placements */,
        silent /*: boolean */,
      ) {
        var keys = placements[element.placement];
        if (!silent && keys.indexOf(element.key) !== -1) {
          throw new TypeError("Duplicated element (" + element.key + ")");
        }
        keys.push(element.key);
      },

      // DecorateElement
      decorateElement: function(
        element /*: ElementDescriptor */,
        placements /*: Placements */,
      ) /*: ElementFinishersExtras */ {
        var extras /*: ElementDescriptor[] */ = [];
        var finishers /*: ClassFinisher[] */ = [];

        for (
          var decorators = element.decorators, i = decorators.length - 1;
          i >= 0;
          i--
        ) {
          // (inlined) RemoveElementPlacement
          var keys = placements[element.placement];
          keys.splice(keys.indexOf(element.key), 1);

          var elementObject /*: ElementObjectInput */ = this.fromElementDescriptor(
            element,
          );
          var elementFinisherExtras /*: ElementFinisherExtras */ = this.toElementFinisherExtras(
            (0, decorators[i])(elementObject) /*: ElementObjectOutput */ ||
              elementObject,
          );

          element = elementFinisherExtras.element;
          this.addElementPlacement(element, placements);

          if (elementFinisherExtras.finisher) {
            finishers.push(elementFinisherExtras.finisher);
          }

          var newExtras /*: ElementDescriptor[] | void */ =
            elementFinisherExtras.extras;
          if (newExtras) {
            for (var j = 0; j < newExtras.length; j++) {
              this.addElementPlacement(newExtras[j], placements);
            }
            extras.push.apply(extras, newExtras);
          }
        }

        return { element: element, finishers: finishers, extras: extras };
      },

      // DecorateConstructor
      decorateConstructor: function(
        elements /*: ElementDescriptor[] */,
        decorators /*: ClassDecorator[] */,
      ) /*: ElementsFinishers */ {
        var finishers /*: ClassFinisher[] */ = [];

        for (var i = decorators.length - 1; i >= 0; i--) {
          var obj /*: ClassObject */ = this.fromClassDescriptor(elements);
          var elementsAndFinisher /*: ElementsFinisher */ = this.toClassDescriptor(
            (0, decorators[i])(obj) /*: ClassObject */ || obj,
          );

          if (elementsAndFinisher.finisher !== undefined) {
            finishers.push(elementsAndFinisher.finisher);
          }

          if (elementsAndFinisher.elements !== undefined) {
            elements = elementsAndFinisher.elements;

            for (var j = 0; j < elements.length - 1; j++) {
              for (var k = j + 1; k < elements.length; k++) {
                if (
                  elements[j].key === elements[k].key &&
                  elements[j].placement === elements[k].placement
                ) {
                  throw new TypeError(
                    "Duplicated element (" + elements[j].key + ")",
                  );
                }
              }
            }
          }
        }

        return { elements: elements, finishers: finishers };
      },

      // FromElementDescriptor
      fromElementDescriptor: function(
        element /*: ElementDescriptor */,
      ) /*: ElementObject */ {
        var obj /*: ElementObject */ = {
          kind: element.kind,
          key: element.key,
          placement: element.placement,
          descriptor: element.descriptor,
        };

        var desc = {
          value: "Descriptor",
          configurable: true,
        };
        Object.defineProperty(obj, Symbol.toStringTag, desc);

        if (element.kind === "field") obj.initializer = element.initializer;

        return obj;
      },

      // ToElementDescriptors
      toElementDescriptors: function(
        elementObjects /*: ElementObject[] */,
      ) /*: ElementDescriptor[] */ {
        if (elementObjects === undefined) return;
        return toArray(elementObjects).map(function(elementObject) {
          var element = this.toElementDescriptor(elementObject);
          this.disallowProperty(elementObject, "finisher", "An element descriptor");
          this.disallowProperty(elementObject, "extras", "An element descriptor");
          return element;
        }, this);
      },

      // ToElementDescriptor
      toElementDescriptor: function(
        elementObject /*: ElementObject */,
      ) /*: ElementDescriptor */ {
        var kind = String(elementObject.kind);
        if (kind !== "method" && kind !== "field") {
          throw new TypeError(
            'An element descriptor\\'s .kind property must be either "method" or' +
              ' "field", but a decorator created an element descriptor with' +
              ' .kind "' +
              kind +
              '"',
          );
        }

        var key = toPropertyKey(elementObject.key);

        var placement = String(elementObject.placement);
        if (
          placement !== "static" &&
          placement !== "prototype" &&
          placement !== "own"
        ) {
          throw new TypeError(
            'An element descriptor\\'s .placement property must be one of "static",' +
              ' "prototype" or "own", but a decorator created an element descriptor' +
              ' with .placement "' +
              placement +
              '"',
          );
        }

        var descriptor /*: PropertyDescriptor */ = elementObject.descriptor;

        this.disallowProperty(elementObject, "elements", "An element descriptor");

        var element /*: ElementDescriptor */ = {
          kind: kind,
          key: key,
          placement: placement,
          descriptor: Object.assign({}, descriptor),
        };

        if (kind !== "field") {
          this.disallowProperty(elementObject, "initializer", "A method descriptor");
        } else {
          this.disallowProperty(
            descriptor,
            "get",
            "The property descriptor of a field descriptor",
          );
          this.disallowProperty(
            descriptor,
            "set",
            "The property descriptor of a field descriptor",
          );
          this.disallowProperty(
            descriptor,
            "value",
            "The property descriptor of a field descriptor",
          );

          element.initializer = elementObject.initializer;
        }

        return element;
      },

      toElementFinisherExtras: function(
        elementObject /*: ElementObject */,
      ) /*: ElementFinisherExtras */ {
        var element /*: ElementDescriptor */ = this.toElementDescriptor(
          elementObject,
        );
        var finisher /*: ClassFinisher */ = _optionalCallableProperty(
          elementObject,
          "finisher",
        );
        var extras /*: ElementDescriptors[] */ = this.toElementDescriptors(
          elementObject.extras,
        );

        return { element: element, finisher: finisher, extras: extras };
      },

      // FromClassDescriptor
      fromClassDescriptor: function(
        elements /*: ElementDescriptor[] */,
      ) /*: ClassObject */ {
        var obj = {
          kind: "class",
          elements: elements.map(this.fromElementDescriptor, this),
        };

        var desc = { value: "Descriptor", configurable: true };
        Object.defineProperty(obj, Symbol.toStringTag, desc);

        return obj;
      },

      // ToClassDescriptor
      toClassDescriptor: function(
        obj /*: ClassObject */,
      ) /*: ElementsFinisher */ {
        var kind = String(obj.kind);
        if (kind !== "class") {
          throw new TypeError(
            'A class descriptor\\'s .kind property must be "class", but a decorator' +
              ' created a class descriptor with .kind "' +
              kind +
              '"',
          );
        }

        this.disallowProperty(obj, "key", "A class descriptor");
        this.disallowProperty(obj, "placement", "A class descriptor");
        this.disallowProperty(obj, "descriptor", "A class descriptor");
        this.disallowProperty(obj, "initializer", "A class descriptor");
        this.disallowProperty(obj, "extras", "A class descriptor");

        var finisher = _optionalCallableProperty(obj, "finisher");
        var elements = this.toElementDescriptors(obj.elements);

        return { elements: elements, finisher: finisher };
      },

      // RunClassFinishers
      runClassFinishers: function(
        constructor /*: Class<*> */,
        finishers /*: ClassFinisher[] */,
      ) /*: Class<*> */ {
        for (var i = 0; i < finishers.length; i++) {
          var newConstructor /*: ?Class<*> */ = (0, finishers[i])(constructor);
          if (newConstructor !== undefined) {
            // NOTE: This should check if IsConstructor(newConstructor) is false.
            if (typeof newConstructor !== "function") {
              throw new TypeError("Finishers must return a constructor.");
            }
            constructor = newConstructor;
          }
        }
        return constructor;
      },

      disallowProperty: function(obj, name, objectType) {
        if (obj[name] !== undefined) {
          throw new TypeError(objectType + " can't have a ." + name + " property.");
        }
      }
    };

    return api;
  }

  // ClassElementEvaluation
  function _createElementDescriptor(
    def /*: ElementDefinition */,
  ) /*: ElementDescriptor */ {
    var key = toPropertyKey(def.key);

    var descriptor /*: PropertyDescriptor */;
    if (def.kind === "method") {
      descriptor = {
        value: def.value,
        writable: true,
        configurable: true,
        enumerable: false,
      };
    } else if (def.kind === "get") {
      descriptor = { get: def.value, configurable: true, enumerable: false };
    } else if (def.kind === "set") {
      descriptor = { set: def.value, configurable: true, enumerable: false };
    } else if (def.kind === "field") {
      descriptor = { configurable: true, writable: true, enumerable: true };
    }

    var element /*: ElementDescriptor */ = {
      kind: def.kind === "field" ? "field" : "method",
      key: key,
      placement: def.static
        ? "static"
        : def.kind === "field"
        ? "own"
        : "prototype",
      descriptor: descriptor,
    };
    if (def.decorators) element.decorators = def.decorators;
    if (def.kind === "field") element.initializer = def.value;

    return element;
  }

  // CoalesceGetterSetter
  function _coalesceGetterSetter(
    element /*: ElementDescriptor */,
    other /*: ElementDescriptor */,
  ) {
    if (element.descriptor.get !== undefined) {
      other.descriptor.get = element.descriptor.get;
    } else {
      other.descriptor.set = element.descriptor.set;
    }
  }

  // CoalesceClassElements
  function _coalesceClassElements(
    elements /*: ElementDescriptor[] */,
  ) /*: ElementDescriptor[] */ {
    var newElements /*: ElementDescriptor[] */ = [];

    var isSameElement = function(
      other /*: ElementDescriptor */,
    ) /*: boolean */ {
      return (
        other.kind === "method" &&
        other.key === element.key &&
        other.placement === element.placement
      );
    };

    for (var i = 0; i < elements.length; i++) {
      var element /*: ElementDescriptor */ = elements[i];
      var other /*: ElementDescriptor */;

      if (
        element.kind === "method" &&
        (other = newElements.find(isSameElement))
      ) {
        if (
          _isDataDescriptor(element.descriptor) ||
          _isDataDescriptor(other.descriptor)
        ) {
          if (_hasDecorators(element) || _hasDecorators(other)) {
            throw new ReferenceError(
              "Duplicated methods (" + element.key + ") can't be decorated.",
            );
          }
          other.descriptor = element.descriptor;
        } else {
          if (_hasDecorators(element)) {
            if (_hasDecorators(other)) {
              throw new ReferenceError(
                "Decorators can't be placed on different accessors with for " +
                  "the same property (" +
                  element.key +
                  ").",
              );
            }
            other.decorators = element.decorators;
          }
          _coalesceGetterSetter(element, other);
        }
      } else {
        newElements.push(element);
      }
    }

    return newElements;
  }

  function _hasDecorators(element /*: ElementDescriptor */) /*: boolean */ {
    return element.decorators && element.decorators.length;
  }

  function _isDataDescriptor(desc /*: PropertyDescriptor */) /*: boolean */ {
    return (
      desc !== undefined &&
      !(desc.value === undefined && desc.writable === undefined)
    );
  }

  function _optionalCallableProperty /*::<T>*/(
    obj /*: T */,
    name /*: $Keys<T> */,
  ) /*: ?Function */ {
    var value = obj[name];
    if (value !== undefined && typeof value !== "function") {
      throw new TypeError("Expected '" + name + "' to be a function");
    }
    return value;
  }

`;

helpers.classPrivateMethodGet = helper("7.1.6")`
  export default function _classPrivateMethodGet(receiver, privateSet, fn) {
    if (!privateSet.has(receiver)) {
      throw new TypeError("attempted to get private field on non-instance");
    }
    return fn;
  }
`;

helpers.classPrivateMethodSet = helper("7.1.6")`
  export default function _classPrivateMethodSet() {
    throw new TypeError("attempted to reassign private method");
  }
`;

helpers.wrapRegExp = helper("7.2.6")`
  import wrapNativeSuper from "wrapNativeSuper";
  import getPrototypeOf from "getPrototypeOf";
  import possibleConstructorReturn from "possibleConstructorReturn";
  import inherits from "inherits";

  export default function _wrapRegExp(re, groups) {
    _wrapRegExp = function(re, groups) {
      return new BabelRegExp(re, groups);
    };

    var _RegExp = wrapNativeSuper(RegExp);
    var _super = RegExp.prototype;
    var _groups = new WeakMap();

    function BabelRegExp(re, groups) {
      var _this = _RegExp.call(this, re);
      _groups.set(_this, groups);
      return _this;
    }
    inherits(BabelRegExp, _RegExp);

    BabelRegExp.prototype.exec = function(str) {
      var result = _super.exec.call(this, str);
      if (result) result.groups = buildGroups(result, this);
      return result;
    };
    BabelRegExp.prototype[Symbol.replace] = function(str, substitution) {
      if (typeof substitution === "string") {
        var groups = _groups.get(this);
        return _super[Symbol.replace].call(
          this,
          str,
          substitution.replace(/\\$<([^>]+)>/g, function(_, name) {
            return "$" + groups[name];
          })
        );
      } else if (typeof substitution === "function") {
        var _this = this;
        return _super[Symbol.replace].call(
          this,
          str,
          function() {
            var args = [];
            args.push.apply(args, arguments);
            if (typeof args[args.length - 1] !== "object") {
              // Modern engines already pass result.groups as the last arg.
              args.push(buildGroups(args, _this));
            }
            return substitution.apply(this, args);
          }
        );
      } else {
        return _super[Symbol.replace].call(this, str, substitution);
      }
    }

    function buildGroups(result, re) {
      // NOTE: This function should return undefined if there are no groups,
      // but in that case Babel doesn't add the wrapper anyway.

      var g = _groups.get(re);
      return Object.keys(groups).reduce(function(groups, name) {
        groups[name] = result[g[name]];
        return groups;
      }, Object.create(null));
    }

    return _wrapRegExp.apply(this, arguments);
  }
`;