Add asserts to catch potential misuses of the rendering framework.

sky/sdk/lib/rendering/block.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';
 
 class BlockParentData extends BoxParentData with ContainerParentDataMixin<RenderBox> { }
@@ skipping 66 matching lines @@
     return _getIntrinsicHeight(constraints);
   }
 
   double getDistanceToActualBaseline(TextBaseline baseline) {
     return defaultGetDistanceToFirstActualBaseline(baseline);
   }
 
   void performLayout() {
     assert(constraints is BoxConstraints);
     double width = constraints.constrainWidth(constraints.maxWidth);
-    bool usesChildSize = !constraints.hasTightHeight;
     BoxConstraints innerConstraints = _getInnerConstraintsForWidth(width);
     double y = 0.0;
     RenderBox child = firstChild;
     while (child != null) {
-      child.layout(innerConstraints, parentUsesSize: usesChildSize);
+      child.layout(innerConstraints, parentUsesSize: true);
       assert(child.parentData is BlockParentData);
       child.parentData.position = new Point(0.0, y);
       y += child.size.height;
       child = child.parentData.nextSibling;
     }
     size = new Size(width, constraints.constrainHeight(y));
     assert(size.width < double.INFINITY);
     assert(size.height < double.INFINITY);
   }
 
   void hitTestChildren(HitTestResult result, { Point position }) {
     defaultHitTestChildren(result, position: position);
   }
 
   void paint(RenderCanvas canvas) {
     defaultPaint(canvas);
   }
 
 }