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

sky/examples/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;
-
-  /// The transform mode used to fit the sprite node tree to the size of the widget.
-  final SpriteBoxTransformMode transformMode;
-
-  /// Creates a new sprite widget with [rootNode] as its content.
-  ///
-  /// The widget will setup the coordinate space for the sprite node tree using the size of the [rootNode] in
-  /// combination with the supplied [transformMode]. By default the letterbox transform mode is used. See
-  /// [SpriteBoxTransformMode] for more details on the different modes.
-  ///
-  /// The most common way to setup the sprite node graph is to subclass [NodeWithSize] and pass it to the sprite widget.
-  /// In the custom subclass it's possible to build the node graph, do animations and handle user events.
-  ///
-  ///     var mySpriteTree = new MyCustomNodeWithSize();
-  ///     var mySpriteWidget = new SpriteWidget(mySpriteTree, SpriteBoxTransformMode.fixedHeight);
-  SpriteWidget(this.rootNode, [this.transformMode = SpriteBoxTransformMode.letterbox]);
-
-  SpriteBox get root => super.root;
-
-  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);
-  }
-}