Fixes build for Safari

Author: satyavh

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

@@ -298,6 +298,9 @@ module.exports = {
           // 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