Specify ecma version (facebook#3743)

Author: Timer

config/webpack.config.prod.js

@@ -293,6 +293,7 @@ module.exports = {
     // Minify the code.
     new UglifyJsPlugin({
       uglifyOptions: {
+        ecma: 5,
         compress: {
           warnings: false,
           // Disabled because of an issue with Uglify breaking seemingly valid code:

package.json

@@ -53,7 +53,7 @@
     "react-dev-utils": "^5.0.0",
     "style-loader": "0.19.0",
     "sw-precache-webpack-plugin": "0.11.4",
-    "uglifyjs-webpack-plugin": "1.1.4",
+    "uglifyjs-webpack-plugin": "1.1.6",
     "url-loader": "0.6.2",
     "webpack": "3.8.1",
     "webpack-dev-server": "2.9.4",