ensure injected css modules computed is a fresh object

Author: yyx990803

lib/component-normalizer.js

@@ -32,11 +32,12 @@ module.exports = function normalizeComponent (
 
   // inject cssModules
   if (cssModules) {
-    var computed = options.computed || (options.computed = {})
+    var computed = Object.create(options.computed || null)
     Object.keys(cssModules).forEach(function (key) {
       var module = cssModules[key]
       computed[key] = function () { return module }
     })
+    options.computed = computed
   }
 
   return {