title: Make SliverAppBar have an image as a background instead of a color
body: <p>I have a SliverAppBar with a background image.</p>
  
  <p>When collapsed it has a blue color as a background:</p>
  
  <p><a href="https://i.stack.imgur.com/LskEcl.png" rel="noreferrer"><img src="https://i.stack.imgur.com/LskEcl.png" alt="enter image description here"></a>
  <a href="https://i.stack.imgur.com/ioR0ql.png" rel="noreferrer"><img src="https://i.stack.imgur.com/ioR0ql.png" alt="enter image description here"></a></p>
  
  <p>But instead of the blue color I want it to show the background image when collapsed:</p>
  
  <p><a href="https://i.stack.imgur.com/dH3X0.png" rel="noreferrer"><img src="https://i.stack.imgur.com/dH3X0.png" alt="enter image description here"></a></p>
  
  <p>How can I achieve this?</p>
  
  <p>I've already tried to give the app bar a transparent background color,
  but it did not work.</p>
  
  <p>Code:</p>
  
  <pre><code>
  void main() =&gt; 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() =&gt; _HomeState();
  }
  
  class _HomeState extends State&lt;Home&gt; {
    @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&amp;cs=tinysrgb&amp;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"),
              ),
            ),
          ));
    }
  }
  </code></pre>