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

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

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'box.dart';
 import 'object.dart';
 
 class StackParentData extends BoxParentData with ContainerParentDataMixin<RenderBox> { }
 
 class RenderStack extends RenderBox with ContainerRenderObjectMixin<RenderBox, StackParentData>,
@@ skipping 13 matching lines @@
   Size getIntrinsicDimensions(BoxConstraints constraints) {
     return constraints.constrain(Size.infinite);
   }
 
   void performLayout() {
     size = constraints.constrain(Size.infinite);
     assert(size.width < double.INFINITY);
     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;
     }
   }
 
   void hitTestChildren(HitTestResult result, { Point position }) {
     defaultHitTestChildren(result, position: position);
   }
 
   void paint(RenderObjectDisplayList canvas) {
     defaultPaint(canvas);
   }
 }