Support for icon theming

sky/sdk/example/fitness/lib/settings.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/widgets/basic.dart';
 import 'package:sky/widgets/switch.dart';
 import 'package:sky/widgets/icon_button.dart';
 import 'package:sky/widgets/material.dart';
 import 'package:sky/widgets/menu_item.dart';
 import 'package:sky/widgets/navigator.dart';
@@ skipping 18 matching lines @@
   bool showModeDialog = false;
 
   void _handleBackupChanged(bool value) {
     if (updater != null)
       updater(backup: value ? BackupMode.enabled : BackupMode.disabled);
   }
 
   Widget buildToolBar() {
     return new ToolBar(
       left: new IconButton(
-        icon: 'navigation/arrow_back_white',
+        icon: "navigation/arrow_back",
         onPressed: navigator.pop),
       center: new Text('Settings')
     );
   }
 
   Widget buildSettingsPane() {
     return new Material(
       type: MaterialType.canvas,
       child: new Container(
         padding: const EdgeDims.symmetric(vertical: 20.0),
@@ skipping 18 matching lines @@
     );
   }
 
   Widget build() {
     return new Scaffold(
       toolbar: buildToolBar(),
       body: buildSettingsPane()
     );
   }
 }