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

sky/sdk/lib/example/mine_digger/mine_digger.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:sky' as sky;
 import 'dart:math';
 
 import 'package:sky/rendering/flex.dart';
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/scaffold.dart';
@@ skipping 353 matching lines @@
   }
 
   Widget build() {
     return game.buildUI();
   }
 }
 
 void main() {
   runApp(new MineDiggerApp());
 }