Allow parsing of ecma 8

Author: Timer

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

@@ -315,7 +315,7 @@ module.exports = {
     // Minify the code.
     new UglifyJsPlugin({
       uglifyOptions: {
-        ecma: 5,
+        ecma: 8,
         compress: {
           warnings: false,
           // Disabled because of an issue with Uglify breaking seemingly valid code: