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

sky/sdk/lib/widgets/theme.dart

Index: sky/sdk/lib/widgets/theme.dart
diff --git a/sky/sdk/lib/widgets/theme.dart b/sky/sdk/lib/widgets/theme.dart
index f2ef760d7581a2b56efc2ba816d3dbc1ceda4fda..5e8c03067ac9707e96cdd640fb1543bcd9198e39 100644
--- a/sky/sdk/lib/widgets/theme.dart
+++ b/sky/sdk/lib/widgets/theme.dart
@@ -2,10 +2,12 @@
 // 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({
@@ -19,7 +21,7 @@ class Theme extends Inherited {
 
   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);