[next] Revert to use ecma 5 in uglifyOptions (facebook#4234)

* Revert to use ecma 5 in uglifyOptions

* remove explicit ecma version from uglifyOptions settings

The defaults are already what we want

* be explicit of where we use ecma: 8 and ecma: 5

config/webpack.config.prod.js

@@ -378,8 +378,15 @@ module.exports = {
     // Minify the code.
     new UglifyJsPlugin({
       uglifyOptions: {
-        ecma: 8,
+        parse: {
+          // we want uglify-js to parse ecma 8 code. However we want it to output
+          // ecma 5 compliant code, to avoid issues with older browsers, this is
+          // whey we put `ecma: 5` to the compress and output section
+          // https://github.com/facebook/create-react-app/pull/4234
+          ecma: 8,
+        },
         compress: {
+          ecma: 5,
           warnings: false,
           // Disabled because of an issue with Uglify breaking seemingly valid code:
           // https://github.com/facebook/create-react-app/issues/2376
@@ -391,6 +398,7 @@ module.exports = {
           safari10: true,
         },
         output: {
+          ecma: 5,
           comments: false,
           // Turned on because emoji and regex is not minified properly using default
           // https://github.com/facebook/create-react-app/issues/2488