message: cache vnodes length
diff: @@ -89,8 +89,9 @@ export function cloneVNode (vnode: VNode): VNode {
  }
  export function cloneVNodes (vnodes: Array<VNode>): Array<VNode> {
  - const res = new Array(vnodes.length)
  - for (let i = 0; i < vnodes.length; i++) {
  + const len = vnodes.length
  + const res = new Array(len)
  + for (let i = 0; i < len; i++) {
  res[i] = cloneVNode(vnodes[i])
  }
  return res
  