Introduce a LeafRenderObjectWrapper abstract class so that RenderObjectWrappers that are leaves don…

sky/sdk/lib/widgets/widget.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:async';
 import 'dart:collection';
 import 'dart:mirrors';
 import 'dart:sky' as sky;
 
 import '../app/view.dart';
@@ skipping 531 matching lines @@
   }
 
   void detachRoot() {
     assert(_ancestor != null);
     assert(root != null);
     _ancestor.detachChildRoot(this);
   }
 
 }
 
+abstract class LeafRenderObjectWrapper extends RenderObjectWrapper {
+
+  LeafRenderObjectWrapper({ String key }) : super(key: key);
+
+  void insertChildRoot(RenderObjectWrapper child, dynamic slot) {
+    assert(false);
+  }
+
+  void detachChildRoot(RenderObjectWrapper child) {
+    assert(false);
+  }
+
+}
+
 abstract class OneChildRenderObjectWrapper extends RenderObjectWrapper {
 
   OneChildRenderObjectWrapper({ String key, Widget child })
     : _child = child, super(key: key);
 
   Widget _child;
   Widget get child => _child;
 
   void syncRenderObject(RenderObjectWrapper old) {
     super.syncRenderObject(old);
@@ skipping 318 matching lines @@
     if (root.parent == null) {
       // we haven't attached it yet
       assert(_container.child == null);
       _container.child = root;
     }
     assert(root.parent == _container);
   }
 
   Widget build() => builder();
 }