fix: In development mode, warns if user tries to Vue.set a property that already exists.

In development mode, warns if user tries to Vue.set a property that already exists. Issue reported
in vuejs#8129. Codepen demonstrating the issue available at
https://codepen.io/chrisvfritz/pen/rvzgBR?editors=1010

fix vuejs#8129

Author: bigtunacan

src/core/observer/index.js

@@ -196,10 +196,15 @@ export function defineReactive (
  * already exist.
  */
 export function set (target: Array<any> | Object, key: any, val: any): any {
-  if (process.env.NODE_ENV !== 'production' &&
-    (isUndef(target) || isPrimitive(target))
-  ) {
-    warn(`Cannot set reactive property on undefined, null, or primitive value: ${(target: any)}`)
+  if (process.env.NODE_ENV !== 'production') {
+    if (isUndef(target) || isPrimitive(target)) {
+      warn(`Cannot set reactive property on undefined, null, or primitive value: ${(target: any)}`)
+    }
+    if (Object.getOwnPropertyDescriptor(target, key) &&
+      (typeof (Object.getOwnPropertyDescriptor(target, key).get) === 'undefined') &&
+      !Array.isArray(target)) {
+      warn(`Cannot enable reactivity on a property that is already defined: ${(key: any)}`)
+    }
   }
   if (Array.isArray(target) && isValidArrayIndex(key)) {
     target.length = Math.max(target.length, key)