title: flutter: no refresh indicator when using RefreshIndicator
body: <p>I added the RefreshIndicator to my page, but there is no indicator visible when pull to refresh. The code is below:</p>
  
  <pre><code>class HomePage extends StatefulWidget {
    HomePage({Key key, this.title}) : super(key: key);
  
    final String title;
  
    @override
    _MyHomePageState createState() =&gt; new _MyHomePageState();
  }
  
  class _MyHomePageState extends State&lt;HomePage&gt; {
  
    @override
    Widget build(BuildContext context) {
      return new Scaffold(
        appBar: new AppBar(
          title: new Text(widget.title),
        ),
        body: LocalGalleryTab(),
      );
    }
  }
  
  class LocalGalleryTab extends StatefulWidget {
    @override
    State&lt;StatefulWidget&gt; createState() {
      return _LocalGalleryState();
    }
  }
  
  class _LocalGalleryState extends State&lt;LocalGalleryTab&gt; {
    @override
    Widget build(BuildContext context) {
      return new Container(child: new Center(
        child: new RefreshIndicator(
          child: Text("Local Gallery"),
          onRefresh: _refreshLocalGallery,
        ),
      ));
    }
  
    Future&lt;Null&gt; _refreshLocalGallery() async{
      print('refreshing stocks...');
  
    }
  }
  </code></pre>
  
  <p>How to use the RefreshIndicator? The flutter <a href="https://docs.flutter.io/flutter/material/RefreshIndicator-class.html" rel="noreferrer">doc</a> does not give much infomation.</p>