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

Per discussion about https://codereview.chromium.org/1093633002 it
appears that we don't have a way around having a parent pointer after
all, so this exposes it.

It also renames setAsChild() to adoptChild() for better readability.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1134593007

sdk/lib/framework/layout.dart

@@ -78,11 +78,11 @@ abstract class RenderNode extends Node {
       child.parentData = new ParentData();
   }
 
-  void setAsChild(RenderNode child) { // only for use by subclasses
+  void adoptChild(RenderNode child) { // only for use by subclasses
     // call this whenever you decide a node is a child
     assert(child != null);
     setupPos(child);
-    super.setAsChild(child);
+    super.adoptChild(child);
   }
   void dropChild(RenderNode child) { // only for use by subclasses
     assert(child != null);
@@ -149,7 +149,7 @@ abstract class ContainerRenderNodeMixin<ChildType extends RenderNode, ParentData
     assert(child != before);
     assert(child != _firstChild);
     assert(child != _lastChild);
-    setAsChild(child);
+    adoptChild(child);
     assert(child.parentData is ParentDataType);
     assert(child.parentData.nextSibling == null);
     assert(child.parentData.previousSibling == null);

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();
   }