如何将一个widget对齐到屏幕顶部，并将另一个widget居中到屏幕中间？

import 'package:flutter/material.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Stack(
          children: <Widget>[
            // 对齐到屏幕顶部的widget
            Positioned(
              top: 0,
              child: Container(
                height: 50,
                color: Colors.blue,
                child: Center(
                  child: Text('顶部Widget'),
                ),
              ),
            ),
            // 居中到屏幕中间的widget
            Center(
              child: Container(
                width: 100,
                height: 100,
                color: Colors.red,
                child: Center(
                  child: Text('中间Widget'),
                ),
              ),
            ),
          ],
        ),
      ),
    );
  }
}