Uglifyjs update for es6 support (#249)

* feat(react-scripts/prod): use new uglifyJsPlugin for ES6 support

this closes #21, #158, #248

* feat(react-scripts/prod): execute uglify plugin parallel + enable caching

packages/react-scripts/config/webpack.config.prod.js

@@ -21,6 +21,7 @@ const ForkTsCheckerWebpackPlugin = require('fork-ts-checker-webpack-plugin');
 const paths = require('./paths');
 const getClientEnvironment = require('./env');
 const TsconfigPathsPlugin = require('tsconfig-paths-webpack-plugin');
+const UglifyJsPlugin = require('uglifyjs-webpack-plugin');
 
 // Webpack uses `publicPath` to determine where the app is being served from.
 // It requires a trailing slash, or the file assets will get an incorrect path.
@@ -296,23 +297,28 @@ module.exports = {
     // Otherwise React will be compiled in the very slow development mode.
     new webpack.DefinePlugin(env.stringified),
     // Minify the code.
-    new webpack.optimize.UglifyJsPlugin({
-      compress: {
-        warnings: false,
-        // Disabled because of an issue with Uglify breaking seemingly valid code:
-        // https://github.com/facebookincubator/create-react-app/issues/2376
-        // Pending further investigation:
-        // https://github.com/mishoo/UglifyJS2/issues/2011
-        comparisons: false,
-      },
-      mangle: {
-        safari10: true,
-      },
-      output: {
-        comments: false,
-        // Turned on because emoji and regex is not minified properly using default
-        // https://github.com/facebookincubator/create-react-app/issues/2488
-        ascii_only: true,
+    new UglifyJsPlugin({
+      parallel: true,
+      cache: true,
+      uglifyOptions: {
+        ecma: 8,
+        compress: {
+          warnings: false,
+          // Disabled because of an issue with Uglify breaking seemingly valid code:
+          // https://github.com/facebookincubator/create-react-app/issues/2376
+          // Pending further investigation:
+          // https://github.com/mishoo/UglifyJS2/issues/2011
+          comparisons: false,
+        },
+        mangle: {
+          safari10: true,
+        },
+        output: {
+          comments: false,
+          // Turned on because emoji and regex is not minified properly using default
+          // https://github.com/facebookincubator/create-react-app/issues/2488
+          ascii_only: true,
+        },
       },
       sourceMap: shouldUseSourceMap,
     }),

packages/react-scripts/package.json

@@ -49,6 +49,7 @@
     "tsconfig-paths-webpack-plugin": "^2.0.0",
     "tslint": "^5.7.0",
     "tslint-react": "^3.2.0",
+    "uglifyjs-webpack-plugin": "^1.1.8",
     "url-loader": "0.6.2",
     "webpack": "3.8.1",
     "webpack-dev-server": "2.9.4",