do not use class for now

Author: yyx990803

src/instance/internal/state.js

@@ -17,6 +17,23 @@ import {
 
 export default function (Vue) {
 
+  /**
+   * Accessor for `$data` property, since setting $data
+   * requires observing the new object and updating
+   * proxied properties.
+   */
+
+  Object.defineProperty(Vue.prototype, '$data', {
+    get () {
+      return this._data
+    },
+    set (newData) {
+      if (newData !== this._data) {
+        this._setData(newData)
+      }
+    }
+  })
+
   /**
    * Setup the scope of an instance, which contains:
    * - observed data

src/instance/vue.js

@@ -24,20 +24,8 @@ import lifecycleAPI from './api/lifecycle'
  * @public
  */
 
-class Vue {
-  constructor (options) {
-    this._init(options)
-  }
-
-  get $data () {
-    return this._data
-  }
-
-  set $data (newData) {
-    if (newData !== this._data) {
-      this._setData(newData)
-    }
-  }
+function Vue (options) {
+  this._init(options)
 }
 
 // install internals