如何将Stack布局设置为屏幕高度的一半？

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        body: Center(
          child: Column(
            children: [
              Expanded(
                flex: 1,
                child: Container(
                  color: Colors.blue,
                  child: Center(
                    child: Text(
                      '上半部分',
                      style: TextStyle(
                        color: Colors.white,
                        fontSize: 20,
                      ),
                    ),
                  ),
                ),
              ),
              Expanded(
                flex: 1,
                child: Container(
                  color: Colors.green,
                  child: Center(
                    child: Text(
                      '下半部分',
                      style: TextStyle(
                        color: Colors.white,
                        fontSize: 20,
                      ),
                    ),
                  ),
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}