Flutter:如何让小部件始终在阴影之上

import 'package:flutter/material.dart';

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

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      home: Scaffold(
        appBar: AppBar(
          title: Text('Shadow Demo'),
        ),
        body: Center(
          child: Stack(
            children: [
              Container(
                width: 200,
                height: 200,
                decoration: BoxDecoration(
                  color: Colors.white,
                  boxShadow: [
                    BoxShadow(
                      color: Colors.grey,
                      blurRadius: 10,
                      spreadRadius: 5,
                    ),
                  ],
                ),
              ),
              Positioned(
                top: 10,
                left: 10,
                child: Container(
                  width: 180,
                  height: 180,
                  decoration: BoxDecoration(
                    color: Colors.blue,
                    borderRadius: BorderRadius.circular(10),
                  ),
                  child: Text(
                    'Widget on top of shadow',
                    style: TextStyle(
                      color: Colors.white,
                      fontSize: 20,
                    ),
                  ),
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}