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

sky/sdk/lib/example/widgets/container.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/rendering/box.dart';
 import 'package:sky/rendering/flex.dart';
 import 'package:sky/widgets/raised_button.dart';
 import 'package:sky/widgets/basic.dart';
 
 class ContainerApp extends App {
@@ skipping 31 matching lines @@
       ],
       direction: FlexDirection.vertical,
       justifyContent: FlexJustifyContent.spaceBetween
     );
   }
 }
 
 void main() {
   runApp(new ContainerApp());
 }