有角度 – 如何将父组件注入子组件？

constructor(private _parent:AppComponent) {}   // child component constructor

我收到以下错误：

EXCEPTION: Cannot resolve all parameters for ChildComponent(?). Make sure they all have valid type or annotations.

我失踪了什么

ChildComponent：

import {Component} from 'angular2/core';
import {AppComponent} from './app.component';

@Component({
  selector: 'child',template: `<p>child</p>`
})
export class ChildComponent {
  constructor(private _parent:AppComponent) {}
}

AppComponent：

import {Component} from 'angular2/core';
import {ChildComponent} from './child.component';

@Component({
  selector: 'my-app',template: `{{title}} <child></child>
  `,directives: [ChildComponent]
})
export class AppComponent {
  title = "Angular 2 - inject parent";
  constructor() { console.clear(); }
}

Plunker