Use package:sky imports consistently

sky/sdk/lib/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 'dart:math' as math;
 
-import 'box.dart';
-import 'object.dart';
+import 'package:sky/rendering/box.dart';
+import 'package:sky/rendering/object.dart';
 
 class StackParentData extends BoxParentData with ContainerParentDataMixin<RenderBox> {
   double top;
   double right;
   double bottom;
   double left;
 
   void merge(StackParentData other) {
     if (other.top != null)
       top = other.top;
@@ skipping 173 matching lines @@
   }
 
   void hitTestChildren(HitTestResult result, { Point position }) {
     defaultHitTestChildren(result, position: position);
   }
 
   void paint(PaintingCanvas canvas, Offset offset) {
     defaultPaint(canvas, offset);
   }
 }