Switch from uglifyjs to terser (facebook#5026)

Uglify is no longer maintained and has bugs not present in Terser (its successor)

config/webpack.config.prod.js

@@ -12,7 +12,7 @@ const autoprefixer = require('autoprefixer');
 const path = require('path');
 const webpack = require('webpack');
 const HtmlWebpackPlugin = require('html-webpack-plugin');
-const UglifyJsPlugin = require('uglifyjs-webpack-plugin');
+const TerserPlugin = require('terser-webpack-plugin');
 const MiniCssExtractPlugin = require('mini-css-extract-plugin');
 const OptimizeCSSAssetsPlugin = require('optimize-css-assets-webpack-plugin');
 const ManifestPlugin = require('webpack-manifest-plugin');
@@ -115,10 +115,10 @@ module.exports = {
   },
   optimization: {
     minimizer: [
-      new UglifyJsPlugin({
-        uglifyOptions: {
+      new TerserPlugin({
+        terserOptions: {
           parse: {
-            // we want uglify-js to parse ecma 8 code. However, we don't want it
+            // we want terser to parse ecma 8 code. However, we don't want it
             // to apply any minfication steps that turns valid ecma 5 code
             // into invalid ecma 5 code. This is why the 'compress' and 'output'
             // sections only apply transformations that are ecma 5 safe

package.json

@@ -64,8 +64,8 @@
     "style-loader": "0.21.0",
     "svgr": "1.9.2",
     "sw-precache-webpack-plugin": "0.11.5",
+    "terser-webpack-plugin": "1.1.0",
     "thread-loader": "1.2.0",
-    "uglifyjs-webpack-plugin": "1.2.5",
     "url-loader": "1.0.1",
     "webpack": "4.19.0",
     "webpack-dev-server": "3.1.7",