Replaced UglifyJS with Terser.

package.json

@@ -56,7 +56,7 @@
     "postcss-loader": "^3.0.0",
     "raw-loader": "^3.1.0",
     "style-loader": "^1.0.0",
-    "uglifyjs-webpack-plugin": "^1.3.0",
+    "terser-webpack-plugin": "^2.2.1",
     "webpack": "^4.39.1",
     "webpack-cli": "^3.3.6"
   },

webpack.config.js

@@ -11,7 +11,7 @@ const path = require( 'path' );
 const webpack = require( 'webpack' );
 const { bundler, styles } = require( '@ckeditor/ckeditor5-dev-utils' );
 const CKEditorWebpackPlugin = require( '@ckeditor/ckeditor5-dev-webpack-plugin' );
-const UglifyJsWebpackPlugin = require( 'uglifyjs-webpack-plugin' );
+const TerserPlugin = require( 'terser-webpack-plugin' );
 
 module.exports = {
 	devtool: 'source-map',
@@ -31,14 +31,15 @@ module.exports = {
 
 	optimization: {
 		minimizer: [
-			new UglifyJsWebpackPlugin( {
+			new TerserPlugin( {
 				sourceMap: true,
-				uglifyOptions: {
+				terserOptions: {
 					output: {
 						// Preserve CKEditor 5 license comments.
 						comments: /^!/
 					}
-				}
+				},
+				extractComments: false
 			} )
 		]
 	},