Flutter实现返回按钮不退出app。

各位大佬求教个问题，怎么实现点手机的实体的返回键不会退出程序只是回到桌面，我现在用WillPopScope监听到了返回键操作然后设置返回无效，但是不知道怎么实现回到桌面。感谢。

 Future<bool> _onWillPop() {
    return Future.value(false);
  }
  Widget _buildTextComposer() {
    return new WillPopScope(
        onWillPop: _onWillPop,
        child: new IconTheme(
        data: new IconThemeData(color: Theme.of(context).accentColor),
        child: new Container(
            margin: const EdgeInsets.symmetric(horizontal: 8.0),
            child: new Row(children: <Widget>[
              new Flexible(
                  child: new TextField(
                controller: _textController,
                onSubmitted: _handleSubmitted,
                decoration: new InputDecoration.collapsed(hintText: '发送消息'),
              )),
              new Container(
                margin: new EdgeInsets.symmetric(horizontal: 4.0),
                child: new IconButton(
                    icon: new Icon(Icons.send),
                    onPressed: () => _handleSubmitted(_textController.text)),
              )
            ]))));
  }

用安卓的原生实现，在MainActivity中增加

 public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK) {
        moveTaskToBack(false);
        return true;
    }
    return super.onKeyDown(keyCode, event);
}

@Override
public void onBackPressed() {
    moveTaskToBack(false);
    super.onBackPressed();

}