Update uglifyjs-webpack-plugin to support prod builds with es6 target or higher

[thetric]

The react-scripts package uses the UglifyJsPlugin version distributed by webpack (webpack.optimize.UglifyJsPlugin) which is quite old and doesn't support the minification of ES6 code (see here).
Since version 1.0.0 the plugin (now under a separate repo) uses a new uglify version that supports ES6 minification. Similar projects like angular-cli are already using it (angular/angular-cli#7610) or are planning to use it (create-react-app in 2.0.0) .

Simply replacing the old plugin with the new one would actually resolve #158 and #21 and bring ES6 prod builds! 🚀

[joshfarrant]

Thanks for this @thetric! Any info on when to expect a release?

[thetric]

Closed in 39d6c62

[0x6a68]

could this be publish?

[hellraisercenobit]

Error is back with latest v6 and es2015 target

`
yarn ng s --prod

Uncaught ReferenceError: Must call super constructor in derived class before accessing 'this' or returning from derived constructor`

[thetric]

Looks like an Angular CLI issue. I guess you have mixed up the repos: angular/angular-cli#7799 (comment)