Introduce RenderNodeWithChildMixin

sky/sdk/lib/framework/fn2.dart

Index: sky/sdk/lib/framework/fn2.dart
diff --git a/sky/sdk/lib/framework/fn2.dart b/sky/sdk/lib/framework/fn2.dart
index 3c71bb903459dce165ecd9ef08c9c683692dfa19..a83535857bbe69f40d63e7c73b6169cb15a69855 100644
--- a/sky/sdk/lib/framework/fn2.dart
+++ b/sky/sdk/lib/framework/fn2.dart
@@ -624,6 +624,28 @@ class FillStackContainer extends OneChildListRenderNodeWrapper {
   }
 }
 
+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;