Material light and dark themes for Sky widgets

sky/sdk/lib/widgets/widget.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 'dart:async';
 import 'dart:collection';
 import 'dart:mirrors';
 import 'dart:sky' as sky;
 
 import '../base/hit_test.dart';
@@ skipping 205 matching lines @@
 }
 
 class ParentDataNode extends TagNode {
   ParentDataNode(Widget child, this.parentData, { String key })
     : super(child, key: key);
   final ParentData parentData;
 }
 
 abstract class _Heir implements Widget {
   Map<Type, Inherited> _traits;
-  Inherited inheritedOfType(Type type) => _traits[type];
+  Inherited inheritedOfType(Type type) => _traits == null ? null : _traits[type];
 
   static _Heir _getHeirAncestor(Widget widget) {
     Widget ancestor = widget;
     while (ancestor != null && !(ancestor is _Heir)) {
       ancestor = ancestor.parent;
     }
     return ancestor as _Heir;
   }
 
   void _updateTraitsFromParent(Widget parent) {
@@ skipping 762 matching lines @@
     if (root.parent == null) {
       // we haven't attached it yet
       assert(_container.child == null);
       _container.child = root;
     }
     assert(root.parent == _container);
   }
 
   Widget build() => builder();
 }