Move examples to //sky/sdk/example

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

 part of sprites;
 
 /// A widget that uses a [SpriteBox] to render a sprite node tree to the screen.
 class SpriteWidget extends OneChildRenderObjectWrapper {
 
   /// The rootNode of the sprite node tree.
   ///
   ///     var node = mySpriteWidget.rootNode;
   final NodeWithSize rootNode;
 
@@ skipping 18 matching lines @@
   SpriteBox createNode() => new SpriteBox(rootNode, transformMode);
 
   void syncRenderObject(SpriteWidget old) {
     super.syncRenderObject(old);
 
     // SpriteBox doesn't allow mutation of these properties
     assert(rootNode == root.rootNode);
     assert(transformMode == root._transformMode);
   }
 }