diff --git a/config/webpack-config-legacy-build.js b/config/webpack-config-legacy-build.js
index d66977b9c9..a7fb3a7977 100644
--- a/config/webpack-config-legacy-build.js
+++ b/config/webpack-config-legacy-build.js
@@ -3,6 +3,28 @@ module.exports = {
   entry: './build/index.js',
   devtool: 'source-map',
   mode: 'production',
+  module: {
+    rules: [
+      {
+        test: /^((?!es2015-)[\s\S])*\.js$/,
+        use: {
+          loader: 'buble-loader',
+          options: {
+            transforms: {dangerousForOf: true},
+          },
+        },
+        include: [
+          path.join(
+            __dirname,
+            '..',
+            'node_modules',
+            '@mapbox',
+            'mapbox-gl-style-spec'
+          ),
+        ],
+      },
+    ],
+  },
   resolve: {
     alias: {
       ol: path.resolve('./build/ol'),