Add asserts to catch potential misuses of the rendering framework.

sky/sdk/lib/widgets/scaffold.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 '../rendering/box.dart';
 import '../rendering/object.dart';
 import '../theme/view_configuration.dart';
 import 'widget.dart';
 
 enum ScaffoldSlots {
@@ skipping 21 matching lines @@
   }
 
   Map<ScaffoldSlots, RenderBox> _slots = new Map<ScaffoldSlots, RenderBox>();
   RenderBox operator[] (ScaffoldSlots slot) => _slots[slot];
   void operator[]= (ScaffoldSlots slot, RenderBox value) {
     RenderBox old = _slots[slot];
     if (old == value)
       return;
     if (old != null)
       dropChild(old);
-    _slots[slot] = value;
-    if (value != null)
+    if (value == null) {
+      _slots.remove(slot);
+    } else {
+      _slots[slot] = value;
       adoptChild(value);
+    }
     markNeedsLayout();
   }
 
   void attachChildren() {
     for (ScaffoldSlots slot in ScaffoldSlots.values) {
       RenderBox box = _slots[slot];
       if (box != null)
         box.attach();
     }
   }
@@ skipping 56 matching lines @@
     }
     if (_slots[ScaffoldSlots.drawer] != null) {
       RenderBox drawer = _slots[ScaffoldSlots.drawer];
       drawer.layout(new BoxConstraints(minWidth: 0.0, maxWidth: size.width, minHeight: size.height, maxHeight: size.height));
       assert(drawer.parentData is BoxParentData);
       drawer.parentData.position = Point.origin;
     }
     if (_slots[ScaffoldSlots.floatingActionButton] != null) {
       RenderBox floatingActionButton = _slots[ScaffoldSlots.floatingActionButton];
       Size area = new Size(size.width - kButtonX, size.height - kButtonY);
-      floatingActionButton.layout(new BoxConstraints.loose(area));
+      floatingActionButton.layout(new BoxConstraints.loose(area), parentUsesSize: true);
       assert(floatingActionButton.parentData is BoxParentData);
       floatingActionButton.parentData.position = (area - floatingActionButton.size).toPoint();
     }
   }
 
   void paint(RenderCanvas canvas) {
     for (ScaffoldSlots slot in [ScaffoldSlots.body, ScaffoldSlots.statusBar, ScaffoldSlots.toolbar, ScaffoldSlots.floatingActionButton, ScaffoldSlots.drawer]) {
       RenderBox box = _slots[slot];
       if (box != null) {
         assert(box.parentData is BoxParentData);
@@ skipping 82 matching lines @@
   void syncRenderObject(Widget old) {
     super.syncRenderObject(old);
     _toolbar = syncChild(_toolbar, old is Scaffold ? old._toolbar : null, ScaffoldSlots.toolbar);
     _body = syncChild(_body, old is Scaffold ? old._body : null, ScaffoldSlots.body);
     _statusBar = syncChild(_statusBar, old is Scaffold ? old._statusBar : null, ScaffoldSlots.statusBar);
     _drawer = syncChild(_drawer, old is Scaffold ? old._drawer : null, ScaffoldSlots.drawer);
     _floatingActionButton = syncChild(_floatingActionButton, old is Scaffold ? old._floatingActionButton : null, ScaffoldSlots.floatingActionButton);
   }
 
 }