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

sky/sdk/lib/widgets/widget.dart

Index: sky/sdk/lib/widgets/widget.dart
diff --git a/sky/sdk/lib/widgets/widget.dart b/sky/sdk/lib/widgets/widget.dart
index 4df951b06f07a6e560912ed2f37118373ff0ed2e..fa1f5739d8befb12908e29e938d4595dd521f46d 100644
--- a/sky/sdk/lib/widgets/widget.dart
+++ b/sky/sdk/lib/widgets/widget.dart
@@ -549,6 +549,20 @@ abstract class RenderObjectWrapper extends Widget {
 
 }
 
+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 })