style: update to ES2015

Author: yyx990803

lib/component-normalizer.js

@@ -1,7 +1,8 @@
 /* globals __VUE_SSR_CONTEXT__ */
 
-// this module is a runtime utility for cleaner component module output and will
-// be included in the final webpack user bundle
+// IMPORTANT: Do NOT use ES2015 features in this file.
+// This module is a runtime utility for cleaner component module output and will
+// be included in the final webpack user bundle.
 
 module.exports = function normalizeComponent (
   rawScriptExports,