code: function defineReactive (vm, key, val) {
    if (key in vm) {
      vm[key] = val
    } else {
      Vue.util.defineReactive(vm, key, val)
    }
  }
docs: Define a reactive property in a given vm if it's not defined
  yet
  
  @param {Vue} vm
  @param {string} key
  @param {*} val