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

sky/sdk/lib/example/rendering/shadowed_box.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';
 
 import 'package:sky/rendering/box.dart';
 import 'package:sky/rendering/flex.dart';
 import 'package:sky/rendering/sky_binding.dart';
 import 'package:sky/theme/colors.dart';
@@ skipping 10 matching lines @@
   var paddedBox = new RenderPadding(
     padding: const EdgeDims.all(50.0),
     child: coloredBox);
   new SkyBinding(root: new RenderDecoratedBox(
     decoration: const BoxDecoration(
       backgroundColor: const Color(0xFFFFFFFF)
     ),
     child: paddedBox
   ));
 }