Support for background images on cards, style demo home

sky/sdk/lib/rendering/box.dart

Index: sky/sdk/lib/rendering/box.dart
diff --git a/sky/sdk/lib/rendering/box.dart b/sky/sdk/lib/rendering/box.dart
index fe7a9c66393a57bccc0d0a938b8b3431ea5196ae..36a3e12f94c49b7a576c1bcaa500619786435d06 100644
--- a/sky/sdk/lib/rendering/box.dart
+++ b/sky/sdk/lib/rendering/box.dart
@@ -966,6 +966,10 @@ class RenderDecoratedBox extends RenderProxyBox {
   BoxDecoration get decoration => _painter.decoration;
   void set decoration (BoxDecoration value) {
     assert(value != null);
+    if (_painter.decoration.backgroundImage != null)
+      _painter.decoration.backgroundImage.removeChangeListener(markNeedsPaint);
+    if (value.backgroundImage != null)
+      value.backgroundImage.addChangeListener(markNeedsPaint);
     if (value == _painter.decoration)
       return;
     _painter.decoration = value;