title: Flutter Drag and Drop ListView hasSize is not true
body: <p>Can someone fire up a quick flutter project and replace main.dart with the following and see what I'm doing wrong? I'm trying to get drag and drop working in ListView.</p>
  
  <p>I'm not even sure this is the right approach so if not, please let me know.</p>
  
  <p>The error I'm getting now is:</p>
  
  <p><strong>Another exception was thrown: 'package:flutter/src/rendering/box.dart': Failed assertion: line 1446 pos 12: 'hasSize': is not true.</strong> </p>
  
  <pre><code>import 'package:flutter/material.dart';
  
  void main() =&gt; runApp(new MyApp());
  
  class MyApp extends StatelessWidget {
    @override
    Widget build(BuildContext context) {
      final title = 'Basic List';
  
      var tile1 = new Material(child:
         new ListTile(
            leading: new Icon(Icons.photo),
            title: new Text('Row 1'),
            trailing: new Icon(Icons.reorder),
  
      ));
  
  
      var tile2 = new Material(
          child:
            new ListTile(
              leading: new Icon(Icons.photo),
              title: new Text('Row 2'),
              trailing: new Icon(Icons.reorder),
      ));
  
  
      return new MaterialApp(
        title: title,
        home: new Scaffold(
          appBar: new AppBar(
            title: new Text(title),
          ),
          body:
          new GestureDetector(
            onVerticalDragStart: startDrag,
            onVerticalDragEnd: endDrag,
            child: new ListView(
              shrinkWrap: true,
              children: [
                    new Flex (
  
                      children: &lt;Widget&gt;[
                        new Flexible(
                          child: new Draggable(child: tile1, feedback: 
  tile1),
                        ),
                        new Flexible(
                          child: new Draggable(child: tile2, feedback: 
  tile2),
                        ),
                      ],
                      direction: Axis.vertical,
                      mainAxisAlignment: MainAxisAlignment.start,
                      mainAxisSize: MainAxisSize.min,
                    ),
              ],
            ),
          ),
        ),
      );
    }
  
    void startDrag(DragStartDetails event) {}
  
    void endDrag(DragEndDetails event) {}
  }
  </code></pre>
  
  <p>Thanks</p>