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

sky/framework/fn.dart

Index: sky/framework/fn.dart
diff --git a/sky/framework/fn.dart b/sky/framework/fn.dart
index 906b7078ae63ad1ed24338ed68a8d2f8c7b9b151..3bc9dbbfcc55561ad643c1b9d3db9744cfbbd9ca 100644
--- a/sky/framework/fn.dart
+++ b/sky/framework/fn.dart
@@ -358,7 +358,8 @@ abstract class SkyElementWrapper extends SkyNodeWrapper {
 
       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;