Introduce RenderNodeWithChildMixin

sky/sdk/lib/framework/fn2.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.
 
 library fn;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:mirrors';
 import 'dart:sky' as sky;
@@ skipping 606 matching lines @@
 
   static List<UINode> _positionNodesToFill(List<UINode> input) {
     if (input == null)
       return null;
     return input.map((node) {
       return new ParentDataNode(node, _fillParentData);
     }).toList();
   }
 }
 
+abstract class OneChildRenderNodeWrapper extends RenderNodeWrapper {
+
+  final UINode child;
+  RenderNodeWithChildMixin root;
+
+  OneChildRenderNodeWrapper({
+    Object key,
+    this.child
+  }) : super(key: key);
+
+  void insert(RenderNodeWrapper child, dynamic slot) {
+    assert(slot == null);
+    root.child = child.root;
+  }
+
+  void _remove() {
+    assert(child != null);
+    removeChild(child);
+    super._remove();
+  }
+}
+
 class TextFragment extends RenderNodeWrapper {
 
   RenderCSSInline root;
   RenderCSSInline createNode() => new RenderCSSInline(this, this.data);
 
   static final TextFragment _emptyText = new TextFragment('');
 
   RenderNodeWrapper get emptyNode => _emptyText;
 
   final String data;
@@ skipping 323 matching lines @@
 
   final int color;
 
   RenderSolidColor root;
   RenderSolidColor createNode() => new RenderSolidColor(color, desiredWidth: 40.0, desiredHeight: 130.0);
 
   static final Rectangle _emptyRectangle = new Rectangle(0);
   RenderNodeWrapper get emptyNode => _emptyRectangle;
 
 }