Add a new Theme widget to control color and text color of apps

sky/sdk/lib/widgets/scaffold.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 '../rendering/box.dart';
 import '../rendering/object.dart';
 import '../theme/view_configuration.dart';
 import 'widget.dart';
 
 enum ScaffoldSlots {
@@ skipping 163 matching lines @@
 
   Widget _toolbar;
   Widget _body;
   Widget _statusBar;
   Widget _drawer;
   Widget _floatingActionButton;
 
   RenderScaffold get root => super.root;
   RenderScaffold createNode() => new RenderScaffold();
 
+  void walkChildren(WidgetTreeWalker walker) {
+    if (_toolbar != null)
+      walker(_toolbar);
+    if (_body != null)
+      walker(_body);
+    if (_statusBar != null)
+      walker(_statusBar);
+    if (_drawer != null)
+      walker(_drawer);
+    if (_floatingActionButton != null)
+      walker(_floatingActionButton);
+  }
+
   void insertChildRoot(RenderObjectWrapper child, ScaffoldSlots slot) {
     root[slot] = child != null ? child.root : null;
   }
 
   void detachChildRoot(RenderObjectWrapper child) {
     final root = this.root; // TODO(ianh): Remove this once the analyzer is cleverer
     assert(root is RenderScaffold);
     assert(root == child.root.parent);
     root.remove(child.root);
     assert(root == this.root); // TODO(ianh): Remove this once the analyzer is cleverer
   }
 
   void remove() {
-    if (_toolbar != null)
-      removeChild(_toolbar);
-    if (_body != null)
-      removeChild(_body);
-    if (_statusBar != null)
-      removeChild(_statusBar);
-    if (_drawer != null)
-      removeChild(_drawer);
-    if (_floatingActionButton != null)
-      removeChild(_floatingActionButton);
+    walkChildren((Widget child) => removeChild(child));
     super.remove();
   }
 
   void syncRenderObject(Widget old) {
     super.syncRenderObject(old);
     _toolbar = syncChild(_toolbar, old is Scaffold ? old._toolbar : null, ScaffoldSlots.toolbar);
     _body = syncChild(_body, old is Scaffold ? old._body : null, ScaffoldSlots.body);
     _statusBar = syncChild(_statusBar, old is Scaffold ? old._statusBar : null, ScaffoldSlots.statusBar);
     _drawer = syncChild(_drawer, old is Scaffold ? old._drawer : null, ScaffoldSlots.drawer);
     _floatingActionButton = syncChild(_floatingActionButton, old is Scaffold ? old._floatingActionButton : null, ScaffoldSlots.floatingActionButton);
   }
 
 }