message: fix(core): avoid observing VNodes
  fix
diff: /* @flow */
  import Dep from './dep'
  +import VNode from '../vdom/vnode'
  import { arrayMethods } from './array'
  import {
  def,
  @@ -104,7 +105,7 @@ function copyAugment (target: Object, src: Object, keys: Array<string>) {
  * or the existing observer if the value already has one.
  */
  export function observe (value: any, asRootData: ?boolean): Observer | void {
  - if (!isObject(value)) {
  + if (!isObject(value) || value instanceof VNode) {
  return
  }
  let ob: Observer | void
  