fix(mp-wexin): getOpenerEventChannel的tree sharking

src/core/runtime/wrapper/create-app.js

@@ -12,16 +12,15 @@ import {
 
 export default function createApp (vm) {
   Vue.prototype.getOpenerEventChannel = function () {
-    switch (__PLATFORM__) {
-      case 'mp-weixin':
+    // 微信小程序使用自身getOpenerEventChannel
+    if (__PLATFORM__ === 'mp-weixin') {
       return this.$scope.getOpenerEventChannel()
-      default :
+    }
     if (!this.__eventChannel__) {
       this.__eventChannel__ = new EventChannel()
     }
     return this.__eventChannel__
   }
-  }
   const callHook = Vue.prototype.__call_hook
   Vue.prototype.__call_hook = function (hook, args) {
     if (hook === 'onLoad' && args && args.__id__) {