examples/widgets/container.dart does not display the buttons

Add a missing Material.

Fixes #1402

examples/widgets/container.dart

@@ -18,8 +18,9 @@ class ContainerApp extends StatelessComponent {
             height: 300.0
           )
         ),
-        new Container(
-          decoration: new BoxDecoration(backgroundColor: const Color(0xFFFFFF00)),
+        new Material(
+          color: const Color(0xFFFFFF00),
+          child: new Container(
             padding: new EdgeDims.symmetric(horizontal: 50.0, vertical: 75.0),
             child: new Row(
               children: <Widget>[
@@ -32,6 +33,7 @@ class ContainerApp extends StatelessComponent {
                 )
               ]
             )
+          )
         ),
         new Flexible(
           child: new Container(