如何在Appbar Flutter中反转RoundBottom

class ReversedRoundBottomAppBar extends AppBar {
  ReversedRoundBottomAppBar({
    Key key,
    Widget title,
    List<Widget> actions,
  }) : super(
          key: key,
          title: title,
          actions: actions,
          shape: RoundedRectangleBorder(
            borderRadius: BorderRadius.vertical(
              bottom: Radius.circular(30.0),
            ),
          ),
        );
}

Scaffold(
  appBar: ReversedRoundBottomAppBar(
    title: Text('My App'),
    actions: [
      IconButton(
        icon: Icon(Icons.search),
        onPressed: () {
          // 执行搜索操作
        },
      ),
    ],
  ),
  body: Container(
    // 页面内容
  ),
);