[Effen] Add a "parent" pointer to Node.

sky/sdk/lib/framework/node.dart

Index: sky/sdk/lib/framework/node.dart
diff --git a/sky/sdk/lib/framework/node.dart b/sky/sdk/lib/framework/node.dart
index ec9a1f94fa9f38d0e8297ea64b30018bc3dcc534..ea51099c43fd4d12e7b6c63a60009ed253a43084 100644
--- a/sky/sdk/lib/framework/node.dart
+++ b/sky/sdk/lib/framework/node.dart
@@ -44,15 +44,21 @@ class Node {
   }
   detachChildren() { } // workaround for lack of inter-class mixins in Dart
 
-  void setAsChild(Node child) { // only for use by subclasses
+  Node _parent;
+  Node get parent => _parent;
+  void adoptChild(Node child) { // only for use by subclasses
     assert(child != null);
+    assert(child._parent == null);
+    child._parent = this;
     if (attached)
       child.attach();
     redepthChild(child);
   }
   void dropChild(Node child) { // only for use by subclasses
     assert(child != null);
+    assert(child._parent == this);
     assert(child.attached == attached);
+    child._parent = null;
     if (attached)
       child.detach();
   }