我有一个带背景图片的SliverAppBar。
当折叠时，它具有蓝色作为背景：

但是，我希望它在折叠时显示背景图像，而不是蓝色：

我如何才能做到这一点？
我已经尝试给予应用程序栏一个透明的背景色，但它没有工作。
验证码：

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  var scrollController = ScrollController();

  @override
  Widget build(BuildContext context) {
    return MaterialApp(
        title: 'Flutter Demo',
        theme: ThemeData(
          primarySwatch: Colors.blue,
        ),
        home: Home());
  }
}

class Home extends StatefulWidget {
  @override
  _HomeState createState() => _HomeState();
}

class _HomeState extends State<Home> {
  @override
  Widget build(BuildContext context) {
    return Scaffold(
        body: DefaultTabController(
          length: 2,
          child: NestedScrollView(
            headerSliverBuilder:
                (BuildContext context, bool innerBoxIsScrolled) {
              return [
                SliverAppBar(
                  expandedHeight: 200.0,
                  floating: false,
                  pinned: true,
                  flexibleSpace: FlexibleSpaceBar(
                      centerTitle: true,
                      title: Text("Collapsing Toolbar",
                          style: TextStyle(
                            color: Colors.white,
                            fontSize: 16.0,
                          )),
                      background: Image.network(
                        "https://images.pexels.com/photos/396547/pexels-photo-396547.jpeg?auto=compress&cs=tinysrgb&h=350",
                        fit: BoxFit.cover,
                      )),
                ),
                SliverPadding(
                  padding: new EdgeInsets.all(16.0),
                  sliver: new SliverList(
                    delegate: new SliverChildListDelegate([
                      TabBar(
                        labelColor: Colors.black87,
                        unselectedLabelColor: Colors.grey,
                        tabs: [
                          new Tab(icon: new Icon(Icons.info), text: "Tab 1"),
                          new Tab(
                              icon: new Icon(Icons.lightbulb_outline),
                              text: "Tab 2"),
                        ],
                      ),
                    ]),
                  ),
                ),
              ];
            },
            body: Center(
              child: Text("Sample text"),
            ),
          ),
        ));
  }
}