Adds a SpriteWidget and simplifies sample game setup

sky/examples/game/lib/sprite_widget.dart

Index: sky/examples/game/lib/sprite_widget.dart
diff --git a/sky/examples/game/lib/sprite_widget.dart b/sky/examples/game/lib/sprite_widget.dart
new file mode 100644
index 0000000000000000000000000000000000000000..b2737bc9d5ed2ef1be35f89c7d77deb6a47ab794
--- /dev/null
+++ b/sky/examples/game/lib/sprite_widget.dart
@@ -0,0 +1,15 @@
+part of sprites;
+
+class SpriteWidget extends OneChildRenderObjectWrapper {
+
+  NodeWithSize _rootNode;
+  SpriteBoxTransformMode _transformMode;
+
+  SpriteWidget(NodeWithSize this._rootNode, [SpriteBoxTransformMode this._transformMode = SpriteBoxTransformMode.letterbox]);

[abarth-chromium, 2015/06/19 00:18:06]

You can remove the "NodeWithSize" annotation.  The compiler is smart enough to
get the type from the member variable declaration.

+
+  SpriteBox get root { return super.root; }

[abarth-chromium, 2015/06/19 00:18:06]

SpriteBox get root => super.root;

+
+  SpriteBox createNode() => new SpriteBox(_rootNode, _transformMode);
+
+  NodeWithSize get rootNode => _rootNode;

[abarth-chromium, 2015/06/19 00:18:06]

You need to implement syncRenderObject.  Take a look at the other subclasses of
OneChildRenderObjectWrapper in basic.dart.

You might want to read
<https://github.com/domokit/mojo/blob/master/sky/sdk/lib/widgets/README.md#state>
if you haven't already.  syncRenderObject is the analog to syncFields for
RenderObjectWrappers.

+}