src/vuewild.js
function defineReactive (vm, key, val) {
  if (key in vm) {
    vm[key] = val
  } else {
    Vue.util.defineReactive(vm, key, val)
  }
}
function defineReactive (vm, key, val) {
  if (key in vm) {
    vm[key] = val
  } else {
    Vue.util.defineReactive(vm, key, val)
  }
}
function defineReactive ( vm , key , val ) { if ( key in vm ) { vm [ key ] = val } else { Vue . util . defineReactive ( vm , key , val ) } }
Define a reactive property in a given vm if it's not defined
yet

@param {Vue} vm
@param {string} key
@param {*} val
Define a reactive property in a given vm if it s not defined yet