Support for background images on cards, style demo home

sky/sdk/lib/widgets/widget.dart

Index: sky/sdk/lib/widgets/widget.dart
diff --git a/sky/sdk/lib/widgets/widget.dart b/sky/sdk/lib/widgets/widget.dart
index 78209f0245206399bbaae0201054816d13cda470..45de29d908ec1c231fdfea1b25425fb417ca1a15 100644
--- a/sky/sdk/lib/widgets/widget.dart
+++ b/sky/sdk/lib/widgets/widget.dart
@@ -278,6 +278,7 @@ typedef void EventListener(sky.Event e);
 class Listener extends TagNode  {
 
   Listener({
+    String key,
     Widget child,
     EventListener onWheel,
     GestureEventListener onGestureFlingCancel,
@@ -305,7 +306,7 @@ class Listener extends TagNode  {
          onPointerUp: onPointerUp,
          custom: custom
        ),
-       super(child);
+       super(child, key: key);
 
   final Map<String, sky.EventListener> listeners;