anchor.diff:      super.merge(other);
     }
   
  +  bool get isPositioned => top != null || right != null || bottom != null || left != null;
  +
     String toString() => '${super.toString()}; top=$top; right=$right; bottom=$bottom, left=$left';
   }
   
  
positive.code:       child.parentData = new StackParentData();
    }
  
    double getMinIntrinsicWidth(BoxConstraints constraints) {
      return constraints.constrainWidth(double.INFINITY);
    }
  
    double getMaxIntrinsicWidth(BoxConstraints constraints) {
      return constraints.constrainWidth(double.INFINITY);
    }
  
    double getMinIntrinsicHeight(BoxConstraints constraints) {
      return constraints.constrainHeight(double.INFINITY);
    }
  
    double getMaxIntrinsicHeight(BoxConstraints constraints) {
      return constraints.constrainHeight(double.INFINITY);
    }
  
    void performLayout() {
      size = constraints.constrain(Size.infinite);
      assert(size.width < double.INFINITY);
      assert(size.height < double.INFINITY);
      BoxConstraints innerConstraints = new BoxConstraints.loose(size);
  
      RenderBox child = firstChild;
      while (child != null) {
        assert(child.parentData is StackParentData);
        StackParentData parentData = child.parentData;
  
        BoxConstraints childConstraints = innerConstraints;
  
        if (parentData.left != null && parentData.right != null)
          childConstraints = childConstraints.applyWidth(parentData.right - parentData.left);
        else if (parentData.left != null)
          childConstraints = childConstraints.applyMaxWidth(size.width - parentData.left);
        else if (parentData.right != null)
          childConstraints = childConstraints.applyMaxWidth(size.width - parentData.right);
  
        if (parentData.top != null && parentData.bottom != null)
          childConstraints = childConstraints.applyHeight(parentData.bottom - parentData.top);
        else if (parentData.top != null)
          childConstraints = childConstraints.applyMaxHeight(size.height - parentData.top);
        else if (parentData.bottom != null)
          childConstraints = childConstraints.applyMaxHeight(size.width - parentData.bottom);
  
        child.layout(childConstraints);
  
        double x = 0.0;
        if (parentData.left != null)
          x = parentData.left;
        else if (parentData.right != null)
          x = size.width - parentData.right - child.size.width;
        assert(x >= 0.0 && x + child.size.width <= size.width);
  
        double y = 0.0;
        if (parentData.top != null)
          y = parentData.top;
        else if (parentData.bottom != null)
          y = size.height - parentData.bottom - child.size.height;
        assert(y >= 0.0 && y + child.size.height <= size.height);
  
        parentData.position = new Point(x, y);
  
        child = child.parentData.nextSibling;
      }
    }
  
  
negative.code: // 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';
  