如何在Flutter中混合多个渐变？

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: Stack(
            children: [
              Positioned(
                top: 0,
                left: 0,
                child: Container(
                  width: 200,
                  height: 200,
                  decoration: BoxDecoration(
                    gradient: LinearGradient(
                      colors: [Colors.red, Colors.blue],
                    ),
                  ),
                ),
              ),
              Positioned(
                bottom: 0,
                right: 0,
                child: Container(
                  width: 200,
                  height: 200,
                  decoration: BoxDecoration(
                    gradient: RadialGradient(
                      colors: [Colors.yellow, Colors.green],
                    ),
                  ),
                ),
              ),
            ],
          ),
        ),
      ),
    );
  }
}