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

sky/sdk/lib/example/widgets/hello_widgets.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';
 
 class HelloWorldApp extends App {
   Widget build() {
     return new Text('Hello, widgets!');
   }
 }
 
 void main() {
   runApp(new HelloWorldApp());
 }