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 ebecd821318de476cf42d006966052b2b5443d97..9652e92ec364d289f0e72f20fc928c82fcda7b2e 100644
--- a/sky/sdk/lib/widgets/widget.dart
+++ b/sky/sdk/lib/widgets/widget.dart
@@ -279,6 +279,7 @@ typedef void EventListener(sky.Event e);
 class Listener extends TagNode  {
 
   Listener({
+    String key,
     Widget child,
     EventListener onWheel,
     GestureEventListener onGestureFlingCancel,
@@ -306,7 +307,7 @@ class Listener extends TagNode  {
          onPointerUp: onPointerUp,
          custom: custom
        ),
-       super(child);
+       super(child, key: key);
 
   final Map<String, sky.EventListener> listeners;