fix(core): fix defineReactive unnecessary getters call

The defineReactive gets the property of the object, only if it has no getter

fix vuejs#7280

src/core/observer/index.js

@@ -61,7 +61,7 @@ export class Observer {
   walk (obj: Object) {
     const keys = Object.keys(obj)
     for (let i = 0; i < keys.length; i++) {
-      defineReactive(obj, keys[i], obj[keys[i]])
+      defineReactive(obj, keys[i])
     }
   }
 
@@ -145,6 +145,9 @@ export function defineReactive (
 
   // cater for pre-defined getter/setters
   const getter = property && property.get
+  if (!getter && arguments.length <= 2) {
+    val = obj[key]
+  }
   const setter = property && property.set
 
   let childOb = !shallow && observe(val)