Cleaned up a few analyzer warnings

sky/sdk/lib/widgets/basic.dart

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 import 'package:vector_math/vector_math.dart';
 
 import '../rendering/block.dart';
 import '../rendering/box.dart';
 import '../rendering/flex.dart';
 import '../rendering/object.dart';
@@ skipping 401 matching lines @@
 
   RenderBox get root => super.root;
   RenderBox createNode() => this.renderBox;
 
   final RenderBox renderBox;
 
   void syncRenderObject(Widget old) {
     super.syncRenderObject(old);
     if (old != null) {
       assert(old is WidgetToRenderBoxAdapter);
-      assert(root == old.renderBox);
+      assert(root == old.root);
     }
   }
 
   void insert(RenderObjectWrapper child, dynamic slot) {
     assert(false);
     // WidgetToRenderBoxAdapter cannot have Widget children; by
     // definition, it is the transition out of the Widget world.
   }
 
 }