[Effen] Make the stock app use the radio button widget so that it's being tested.

sky/framework/fn.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 library fn;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:sky' as sky;
 import 'reflect.dart' as reflect;
@@ skipping 340 matching lines @@
   bool _debugHasDuplicateIds() {
     var idSet = new HashSet<String>();
     for (var child in children) {
       assert(child != null);
       if (child is Text) {
         continue; // Text nodes all have the same key and are never reordered.
       }
 
       if (!idSet.add(child._key)) {
         throw '''If multiple (non-Text) nodes of the same type exist as children
-                 of another node, they must have unique keys.''';
+                of another node, they must have unique keys.
+                Duplicate: "${child._key}"''';
       }
     }
     return false;
   }
 
   void _syncNode(SkyNodeWrapper old) {
     SkyElementWrapper oldSkyElementWrapper = old as SkyElementWrapper;
 
     List<Style> styles = new List<Style>();
     if (style != null)
@@ skipping 481 matching lines @@
   }
 
   void _buildIfDirty() {
     if (!_dirty || _defunct)
       return;
 
     _trace('$_key rebuilding...');
     _sync(null, _host, _root);
   }
 }