chore: bundle UMD with rollup (#449)

* Bundle UMD with rollup

```diff
-Webpack production bundle is 18407b (no gzip)
+Rollup production bundle is 14975b (no gzip)
```

* Fix lint

* Compile to lib/dist

* Build for production instead

* Uncomment terser

.size-snapshot.json

@@ -0,0 +1,12 @@
+{
+  "./lib/dist/react-transition-group.js": {
+    "bundled": 79410,
+    "minified": 22589,
+    "gzipped": 6905
+  },
+  "./lib/dist/react-transition-group.min.js": {
+    "bundled": 46139,
+    "minified": 14975,
+    "gzipped": 4685
+  }
+}

package.json

@@ -9,7 +9,7 @@
     "tdd": "jest --watch",
     "build": "babel src --out-dir lib --delete-dir-on-start && npm run build:dist && cp README.md LICENSE ./lib",
     "build:docs": "npm -C www run build",
-    "build:dist": "webpack --mode production",
+    "build:dist": "cross-env BABEL_ENV=esm yarn rollup -c",
     "bootstrap": "yarn && yarn --cwd www",
     "lint": "eslint src test",
     "release": "release",
@@ -21,13 +21,6 @@
     "travis-deploy-once": "travis-deploy-once",
     "semantic-release": "semantic-release"
   },
-  "size-limit": [
-    {
-      "gzip": false,
-      "path": "lib/index.js",
-      "limit": "16.66 KB"
-    }
-  ],
   "repository": {
     "type": "git",
     "url": "https://github.com/reactjs/react-transition-group.git"
@@ -81,6 +74,7 @@
     "babel-loader": "^8.0.2",
     "babel-plugin-transform-react-remove-prop-types": "^0.4.18",
     "babel-preset-jason": "^6.0.1",
+    "cross-env": "^5.2.0",
     "enzyme": "^3.6.0",
     "enzyme-adapter-react-16": "^1.5.0",
     "eslint": "^5.6.0",
@@ -98,14 +92,17 @@
     "react-test-renderer": "^16.5.2",
     "release-script": "^1.0.2",
     "rimraf": "^2.6.1",
+    "rollup": "^1.1.0",
+    "rollup-plugin-babel": "^4.3.0",
+    "rollup-plugin-commonjs": "^9.2.0",
+    "rollup-plugin-node-resolve": "^4.0.0",
+    "rollup-plugin-replace": "^2.1.0",
+    "rollup-plugin-size-snapshot": "^0.8.0",
+    "rollup-plugin-terser": "^4.0.2",
     "semantic-release": "^15.9.16",
     "semantic-release-alt-publish-dir": "^2.1.1",
     "sinon": "^6.3.4",
-    "size-limit": "^0.21.1",
-    "travis-deploy-once": "^5.0.8",
-    "webpack": "^4.19.1",
-    "webpack-atoms": "^8.0.0",
-    "webpack-cli": "^3.1.1"
+    "travis-deploy-once": "^5.0.8"
   },
   "release": {
     "pkgRoot": "lib",

rollup.config.js

@@ -0,0 +1,64 @@
+import nodeResolve from "rollup-plugin-node-resolve";
+import babel from "rollup-plugin-babel";
+import commonjs from "rollup-plugin-commonjs";
+import replace from "rollup-plugin-replace";
+import { sizeSnapshot } from "rollup-plugin-size-snapshot";
+import { terser } from "rollup-plugin-terser";
+
+const input = "./src/umd.js";
+const name = "ReactTransitionGroup";
+const globals = {
+  react: "React",
+  "react-dom": "ReactDOM"
+};
+
+const babelOptions = {
+  exclude: /node_modules/,
+  runtimeHelpers: true
+}
+
+const commonjsOptions = {
+  include: /node_modules/,
+  namedExports: {
+    "prop-types": ["object", "oneOfType", "element", "bool", "func"]
+  }
+};
+
+export default [
+  {
+    input,
+    output: {
+      file: "./lib/dist/react-transition-group.js",
+      format: "umd",
+      name,
+      globals
+    },
+    external: Object.keys(globals),
+    plugins: [
+      nodeResolve(),
+      babel(babelOptions),
+      commonjs(commonjsOptions),
+      replace({ "process.env.NODE_ENV": JSON.stringify("development") }),
+      sizeSnapshot()
+    ]
+  },
+
+  {
+    input,
+    output: {
+      file: "./lib/dist/react-transition-group.min.js",
+      format: "umd",
+      name,
+      globals
+    },
+    external: Object.keys(globals),
+    plugins: [
+      nodeResolve(),
+      babel(babelOptions),
+      commonjs(commonjsOptions),
+      replace({ "process.env.NODE_ENV": JSON.stringify("production") }),
+      sizeSnapshot(),
+      terser()
+    ]
+  }
+];

src/umd.js

@@ -0,0 +1,4 @@
+export { default as CSSTransition } from './CSSTransition';
+export { default as ReplaceTransition } from './ReplaceTransition';
+export { default as TransitionGroup } from './TransitionGroup';
+export { default as Transition } from './Transition';