如何实现一个拱形的appBar

import 'package:flutter/material.dart';

class ArchedAppBarPage extends StatefulWidget {
  @override
  _ArchedAppBarPageState createState() => _ArchedAppBarPageState();
}

class _ArchedAppBarPageState extends State<ArchedAppBarPage> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text('Arched AppBar'),
      ),
      body: CustomScrollView(
        slivers: <Widget>[
          SliverAppBar(
            expandedHeight: 200,
            flexibleSpace: FlexibleSpaceBar(
              background: Container(
                decoration: BoxDecoration(
                  borderRadius: BorderRadius.vertical(
                    bottom: Radius.circular(20),
                  ),
                  gradient: LinearGradient(
                    colors: [Colors.blue, Colors.green],
                    begin: Alignment.topLeft,
                    end: Alignment.bottomRight,
                  ),
                ),
              ),
            ),
          ),
          SliverList(
            delegate: SliverChildBuilderDelegate(
              (context, index) => ListTile(
                title: Text('Item $index'),
              ),
              childCount: 20,
            ),
          ),
        ],
      ),
    );
  }
}