Add Point.origin for new Point(0.0, 0.0)

sky/sdk/lib/framework/rendering/stack.dart

Index: sky/sdk/lib/framework/rendering/stack.dart
diff --git a/sky/sdk/lib/framework/rendering/stack.dart b/sky/sdk/lib/framework/rendering/stack.dart
index b22cb69e5224296c07481eeb2500aaec7e68ede7..7656ea1c702145162fc3753ca71ffa9c04f5f84c 100644
--- a/sky/sdk/lib/framework/rendering/stack.dart
+++ b/sky/sdk/lib/framework/rendering/stack.dart
@@ -31,12 +31,11 @@ class RenderStack extends RenderBox with ContainerRenderObjectMixin<RenderBox, S
     assert(size.height < double.INFINITY);
     BoxConstraints innerConstraints = new BoxConstraints.loose(size);
 
-    Point origin = new Point(0.0, 0.0);
     RenderBox child = firstChild;
     while (child != null) {
       child.layout(innerConstraints);
       assert(child.parentData is StackParentData);
-      child.parentData.position = origin;
+      child.parentData.position = Point.origin;
       child = child.parentData.nextSibling;
     }
   }