When syncing a child, we have to update the parent in all cases.

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 'dart:sky' as sky;
 
 import '../rendering/box.dart';
 import '../rendering/object.dart';
 import '../theme/view_configuration.dart';
 import 'widget.dart';
@@ skipping 163 matching lines @@
 
   Scaffold({
     String key,
     Widget body,
     Widget statusBar,
     Widget toolbar,
     Widget snackBar,
     Widget floatingActionButton,
     Widget drawer
   }) : super(key: key) {
-    this[ScaffoldSlots.body] = body;
-    this[ScaffoldSlots.statusBar] = statusBar;
-    this[ScaffoldSlots.toolbar] = toolbar;
-    this[ScaffoldSlots.snackBar] = snackBar;
-    this[ScaffoldSlots.floatingActionButton] = floatingActionButton;
-    this[ScaffoldSlots.drawer] = drawer;
+    _slots[ScaffoldSlots.body] = body;
+    _slots[ScaffoldSlots.statusBar] = statusBar;
+    _slots[ScaffoldSlots.toolbar] = toolbar;
+    _slots[ScaffoldSlots.snackBar] = snackBar;
+    _slots[ScaffoldSlots.floatingActionButton] = floatingActionButton;
+    _slots[ScaffoldSlots.drawer] = drawer;
   }
 
   Map<ScaffoldSlots, Widget> _slots = new Map<ScaffoldSlots, Widget>();
-  Widget operator[] (ScaffoldSlots slot) => _slots[slot];
-  void operator[]= (ScaffoldSlots slot, Widget value) {
-    _slots[slot] = value;
-  }
 
   RenderScaffold get root => super.root;
   RenderScaffold createNode() => new RenderScaffold();
 
   void walkChildren(WidgetTreeWalker walker) {
     for (ScaffoldSlots slot in ScaffoldSlots.values) {
       Widget widget = _slots[slot];
       if (widget != null)
         walker(widget);
     }
@@ skipping 12 matching lines @@
   }
 
   void remove() {
     walkChildren((Widget child) => removeChild(child));
     super.remove();
   }
 
   void syncRenderObject(Widget old) {
     super.syncRenderObject(old);
     for (ScaffoldSlots slot in ScaffoldSlots.values) {
-      Widget widget = this[slot];
-      this[slot] = syncChild(widget, old is Scaffold ? old[slot] : null, slot);
+      Widget widget = _slots[slot];
+      _slots[slot] = syncChild(widget, old is Scaffold ? old._slots[slot] : null, slot);
+      assert((_slots[slot] == null) == (widget == null));
+      assert(_slots[slot] == null || _slots[slot].parent == this);
     }
   }
 
 }