[Effen] Now that we have RenderNode.parent again, we can have mid-tree sync() work properly again.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1145623003

sdk/lib/framework/fn.dart

@@ -886,7 +886,7 @@ abstract class Component extends UINode {
       return;
 
     _trace('$_key rebuilding...');
-    _sync(null, null, null); // TODO(ianh): figure out how passing "null, null, null" here is ok
+    _sync(null, _root.parent, _root.parent.childAfter(_root));
   }
 
   void scheduleBuild() {