Add asserts to catch potential misuses of the rendering framework.

sky/sdk/lib/rendering/stack.dart

Index: sky/sdk/lib/rendering/stack.dart
diff --git a/sky/sdk/lib/rendering/stack.dart b/sky/sdk/lib/rendering/stack.dart
index be73d6f1aa4e4dd071f53d1f2c63b2601fcd18c7..33563e61bf9ea72aef3808f9aacbb4254250b0f5 100644
--- a/sky/sdk/lib/rendering/stack.dart
+++ b/sky/sdk/lib/rendering/stack.dart
@@ -150,45 +150,45 @@ class RenderStack extends RenderBox with ContainerRenderObjectMixin<RenderBox, S
     child = firstChild;
     while (child != null) {
       assert(child.parentData is StackParentData);
-      final StackParentData parentData = child.parentData;
+      final StackParentData childData = child.parentData;
 
-      if (parentData.isPositioned) {
+      if (childData.isPositioned) {
         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 (childData.left != null && childData.right != null)
+          childConstraints = childConstraints.applyWidth(childData.right - childData.left);
+        else if (childData.left != null)
+          childConstraints = childConstraints.applyMaxWidth(size.width - childData.left);
+        else if (childData.right != null)
+          childConstraints = childConstraints.applyMaxWidth(size.width - childData.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);
+        if (childData.top != null && childData.bottom != null)
+          childConstraints = childConstraints.applyHeight(childData.bottom - childData.top);
+        else if (childData.top != null)
+          childConstraints = childConstraints.applyMaxHeight(size.height - childData.top);
+        else if (childData.bottom != null)
+          childConstraints = childConstraints.applyMaxHeight(size.width - childData.bottom);
 
-        child.layout(childConstraints);
+        child.layout(childConstraints, parentUsesSize: true);
 
         double x = 0.0;
-        if (parentData.left != null)
-          x = parentData.left;
-        else if (parentData.right != null)
-          x = size.width - parentData.right - child.size.width;
+        if (childData.left != null)
+          x = childData.left;
+        else if (childData.right != null)
+          x = size.width - childData.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;
+        if (childData.top != null)
+          y = childData.top;
+        else if (childData.bottom != null)
+          y = size.height - childData.bottom - child.size.height;
         assert(y >= 0.0 && y + child.size.height <= size.height);
 
-        parentData.position = new Point(x, y);
+        childData.position = new Point(x, y);
       }
 
-      child = parentData.nextSibling;
+      child = childData.nextSibling;
     }
   }