Flutter位置堆栈小部件在中心

我有一个堆栈中的小部件，所以我想将我的按钮栏放在堆栈的底部中心，但是什么都不起作用.小部件只会粘在左侧.这是我的代码.

new Positioned(
            bottom: 40.0,
            child: new Container(
              margin: const EdgeInsets.all(16.0),
              child: new Column(
                mainAxisAlignment: MainAxisAlignment.center,
                children: <Widget>[
                  new Align(
                    alignment: Alignment.bottomCenter,
                    child: new ButtonBar(
                      alignment: MainAxisAlignment.center,
                      children: <Widget>[
                        new OutlineButton(
                          onPressed: () {
                            Navigator.push(
                                context,
                                new MaterialPageRoute(
                                    builder: (context) => new LoginPage()));
                          },
                          child: new Text(
                            "Login",
                            style: new TextStyle(color: Colors.white),
                          ),
                        ),
                        new RaisedButton(
                          color: Colors.white,
                          onPressed: () {
                            Navigator.push(
                                context,
                                new MaterialPageRoute(
                                    builder: (context) =>
                                        new RegistrationPage()));
                          },
                          child: new Text(
                            "Register",
                            style: new TextStyle(color: Colors.black),
                          ),
                        )
                      ],
                    ),
                  )
                ],
              ),
            ),
          )

我真的试过了每种居中路由，请帮帮忙.