为什么横轴突出不在flex、row和column中工作?

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(
          'Flutter Demo'
        ),
      ),
      body: Row(
        mainAxisAlignment: MainAxisAlignment.center,
        crossAxisAlignment: CrossAxisAlignment.center,
        children: <Widget>[
          Text(
            'Hell World!'
          )
        ],
      )
    );
  }
}

文本为何 Hell World 只能水平居中,不能垂直居中 CrossAxisAligment .

是因为这个吗? Issue