message: warn computed properties that are already defined as data or props
diff: @@ -165,6 +165,12 @@ function initComputed (vm: Component, computed: Object) {
  // at instantiation here.
  if (!(key in vm)) {
  defineComputed(vm, key, userDef)
  + } else if (process.env.NODE_ENV !== 'production') {
  + if (key in vm.$data) {
  + warn(`The computed property "${key}" is already defined in data.`, vm)
  + } else if (vm.$options.props && key in vm.$options.props) {
  + warn(`The computed property "${key}" is already defined as a prop.`, vm)
  + }
  }
  }
  }
  