code: function dependArray (value) {
    for (let e, i = 0, l = value.length; i < l; i++) {
      e = value[i]
      e && e.__ob__ && e.__ob__.dep.depend()
      if (Array.isArray(e)) {
        dependArray(e)
      }
    }
  }
docs: Collect dependencies on array elements when the array is touched, since
  we cannot intercept array element access like property getters.