Move sky/examples to sky/sdk/lib/example, and code changes to support that change. Fixes T277.

sky/tests/examples/tabs.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 'package:sky/widgets/widget.dart';
 
-import '../../examples/widgets/tabs.dart';
+import '../../sdk/lib/example/widgets/tabs.dart';
 import '../resources/display_list.dart';
 
 main() async {
   TestRenderView testRenderView = new TestRenderView();
   TabbedNavigatorApp app = new TabbedNavigatorApp();
   runApp(app, renderViewOverride: testRenderView);
   await testRenderView.checkFrame();
   testRenderView.endTest();
 }