Teach Sky buttons and dialogs how to use the new Theme system

sky/sdk/lib/widgets/theme.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:sky/theme/theme_data.dart';
+import '../theme/theme_data.dart';
 import 'basic.dart';
 import 'widget.dart';
 
+export '../theme/theme_data.dart' show ThemeBrightness;
+
 class Theme extends Inherited {
 
   Theme({
     String key,
     this.data,
     Widget child
   }) : super(key: key, child: child) {
     assert(child != null);
     assert(data != null);
   }
 
   final ThemeData data;
 
-  static ThemeData _kFallbackTheme = new ThemeData.fallback();
+  static final ThemeData _kFallbackTheme = new ThemeData.fallback();
 
   static ThemeData of(Component component) {
     Theme theme = component.inheritedOfType(Theme);
     return theme == null ? _kFallbackTheme : theme.data;
   }
 }