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

sky/sdk/lib/example/game/main.dart

 import 'dart:sky';
 
 import 'package:sky/widgets/basic.dart';
 import 'package:sky/widgets/raised_button.dart';
 import 'package:sky/widgets/widget.dart';
 import 'package:sky/framework/net/fetch.dart';
 
 import 'lib/game_demo.dart';
 import 'lib/sprites.dart';
 
@@ skipping 38 matching lines @@
 //        ]),
 //        right:0.0,
 //        top: 20.0
 //      )
     ]);
   }
 }
 
 ImageMap _loader;
 SpriteSheet _spriteSheet;