Add asserts to catch potential misuses of the rendering framework.

sky/sdk/lib/rendering/block.dart

Index: sky/sdk/lib/rendering/block.dart
diff --git a/sky/sdk/lib/rendering/block.dart b/sky/sdk/lib/rendering/block.dart
index 3ab31711dfecd20e391903a295207dca87d4cdf8..4cceb68a9927247a2bba7bf06b0fc43aa9ef7ea9 100644
--- a/sky/sdk/lib/rendering/block.dart
+++ b/sky/sdk/lib/rendering/block.dart
@@ -84,12 +84,11 @@ class RenderBlock extends RenderBox with ContainerRenderObjectMixin<RenderBox, B
   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;