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

sky/sdk/lib/framework/node.dart

 library node;
 
 class Node {
 
   // Nodes always have a 'depth' greater than their ancestors'.
   // There's no guarantee regarding depth between siblings. The depth
   // of a node is used to ensure that nodes are processed in depth
   // order. The 'depth' of a child can be more than one greater than
   // the 'depth' of the parent, because the 'depth' values are never
   // decreased: all that matters is that it's greater than the parent.
@@ skipping 26 matching lines @@
   }
   attachChildren() { } // workaround for lack of inter-class mixins in Dart
   void detach() {
     // override this in subclasses with child nodes
     // simply call detach() for each child then call your superclass
     _attached = false;
     detachChildren();
   }
   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();
   }
 
 }