Disable Uglify reduce_vars (#2200)

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

@@ -280,6 +280,10 @@ module.exports = {
     new webpack.optimize.UglifyJsPlugin({
       compress: {
         warnings: false,
+        // This feature has been reported as buggy a few times, such as:
+        // https://github.com/mishoo/UglifyJS2/issues/1964
+        // We'll wait with enabling it by default until it is more solid.
+        reduce_vars: false,
       },
       output: {
         comments: false,