Revert "Throw error for multiple exports default (babel#3518)"

This reverts commit aa51dd4.

packages/babel-plugin-transform-es2015-modules-amd/test/fixtures/amd/exports-default/actual.js

@@ -0,0 +1,10 @@
+export default 42;
+export default {};
+export default [];
+export default foo;
+export default function () {}
+export default class {}
+export default function foo () {}
+export default class Foo {}
+export { foo as default };
+export default (function(){return "foo"})();

packages/babel-plugin-transform-es2015-modules-amd/test/fixtures/amd/exports-default/expected.js

@@ -0,0 +1,24 @@
+define(["exports"], function (exports) {
+  "use strict";
+
+  Object.defineProperty(exports, "__esModule", {
+    value: true
+  });
+
+  exports.default = function () {};
+
+  exports.default = foo;
+  exports.default = 42;
+  exports.default = {};
+  exports.default = [];
+  exports.default = foo;
+  exports.default = class {};
+  function foo() {}
+  class Foo {}
+  exports.default = Foo;
+  exports.default = foo;
+
+  exports.default = function () {
+    return "foo";
+  }();
+});

packages/babel-plugin-transform-es2015-modules-commonjs/src/index.js

@@ -170,8 +170,6 @@ export default function () {
 
           let requires = Object.create(null);
 
-          let exportDefaultFound = false;
-
           function addRequire(source, blockHoist) {
             let cached = requires[source];
             if (cached) return cached;
@@ -240,10 +238,6 @@ export default function () {
 
               path.remove();
             } else if (path.isExportDefaultDeclaration()) {
-              if (exportDefaultFound) {
-                throw path.buildCodeFrameError("Only one default export allowed per module.");
-              }
-
               let declaration = path.get("declaration");
               if (declaration.isFunctionDeclaration()) {
                 let id = declaration.node.id;
@@ -281,7 +275,6 @@ export default function () {
                 // replaceWith above. See #4140 for more info.
                 path.parentPath.requeue(path.get("expression.left"));
               }
-              exportDefaultFound = true;
             } else if (path.isExportNamedDeclaration()) {
               let declaration = path.get("declaration");
               if (declaration.node) {

packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/interop/exports-default/actual.js

@@ -0,0 +1,10 @@
+export default 42;
+export default {};
+export default [];
+export default foo;
+export default function () {}
+export default class {}
+export default function foo () {}
+export default class Foo {}
+export { foo as default };
+export default (function(){return "foo"})();

packages/babel-plugin-transform-es2015-modules-commonjs/test/fixtures/interop/exports-default/expected.js

@@ -0,0 +1,22 @@
+"use strict";
+
+Object.defineProperty(exports, "__esModule", {
+  value: true
+});
+
+exports.default = function () {};
+
+exports.default = foo;
+exports.default = 42;
+exports.default = {};
+exports.default = [];
+exports.default = foo;
+exports.default = class {};
+function foo() {}
+class Foo {}
+exports.default = Foo;
+exports.default = foo;
+
+exports.default = function () {
+  return "foo";
+}();