switch from preset-env to preset-es2015

package.json

@@ -19,7 +19,7 @@
     "babel": "^6.23.0",
     "babel-core": "^6.26.0",
     "babel-loader": "^7.1.2",
-    "babel-preset-env": "^1.6.1",
+    "babel-preset-es2015": "^6.24.1",
     "compression-webpack-plugin": "^1.0.1",
     "file-loader": "^1.1.5",
     "mithril": "^1.1.5",

webpack.config.js

@@ -36,7 +36,7 @@ const config = {
         exclude: /node_modules/,
         use: [{
           loader: 'babel-loader',
-          options: { presets: ['env'] },
+          options: { presets: ['es2015'] },
         }],
       },
       {