Move examples to //sky/sdk/example

sky/sdk/lib/example/hello_world/hello_world.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/framework/fn.dart';
-
-class HelloWorldApp extends App {
-  UINode build() {
-    return new Text('Hello, world!');
-  }
-}