Adds a SpriteWidget and simplifies sample game setup

sky/examples/game/lib/game_demo_box.dart

Index: sky/examples/game/lib/game_demo_box.dart
diff --git a/sky/examples/game/lib/game_demo_box.dart b/sky/examples/game/lib/game_demo_box.dart
deleted file mode 100644
index 2b9f0c4d325651b3ec27e0231ee9bc46418af3e0..0000000000000000000000000000000000000000
--- a/sky/examples/game/lib/game_demo_box.dart
+++ /dev/null
@@ -1,79 +0,0 @@
-part of game;
-
-const double _steeringThreshold = 0.0;
-const double _steeringMax = 150.0;
-
-class GameDemoBox extends SpriteBox {
-
-  GameDemoBox(GameDemoWorld game) : super(game, SpriteBoxTransformMode.letterbox);
-
-  GameDemoWorld get _gameWorld => this.rootNode;
-
-  // Handle pointers
-  int _firstPointer = -1;
-  int _secondPointer = -1;
-  Point _firstPointerDownPos;
-
-//  void handleEvent(Event event, BoxHitTestEntry entry) {
-//    if (event is PointerEvent) {
-//      Point pointerPos = new Point(event.x, event.y);
-//      int pointer = event.pointer;
-//
-//      switch (event.type) {
-//        case 'pointerdown':
-//          if (_firstPointer == -1) {
-//            // Assign the first pointer
-//            _firstPointer = pointer;
-//            _firstPointerDownPos = pointerPos;
-//          }
-//          else if (_secondPointer == -1) {
-//            // Assign second pointer
-//            _secondPointer = pointer;
-//            _gameWorld.controlFire();
-//          }
-//          else {
-//            // There is a pointer used for steering, let's fire instead
-//            _gameWorld.controlFire();
-//          }
-//          break;
-//        case 'pointermove':
-//          if (pointer == _firstPointer) {
-//            // Handle turning control
-//            double joystickX = 0.0;
-//            double deltaX = pointerPos.x - _firstPointerDownPos.x;
-//            if (deltaX > _steeringThreshold || deltaX < -_steeringThreshold) {
-//              joystickX = (deltaX - _steeringThreshold)/(_steeringMax - _steeringThreshold);
-//              if (joystickX > 1.0) joystickX = 1.0;
-//              if (joystickX < -1.0) joystickX = -1.0;
-//            }
-//
-//            double joystickY = 0.0;
-//            double deltaY = pointerPos.y - _firstPointerDownPos.y;
-//            if (deltaY > _steeringThreshold || deltaY < -_steeringThreshold) {
-//              joystickY = (deltaY - _steeringThreshold)/(_steeringMax - _steeringThreshold);
-//              if (joystickY > 1.0) joystickY = 1.0;
-//              if (joystickY < -1.0) joystickY = -1.0;
-//            }
-//
-//            _gameWorld.controlSteering(joystickX, joystickY);
-//          }
-//          break;
-//        case 'pointerup':
-//        case 'pointercancel':
-//          if (pointer == _firstPointer) {
-//            // Un-assign the first pointer
-//            _firstPointer = -1;
-//            _firstPointerDownPos = null;
-//            _gameWorld.controlSteering(0.0, 0.0);
-//          }
-//          else if (pointer == _secondPointer) {
-//            _secondPointer = -1;
-//          }
-//          break;
-//        default:
-//          break;
-//      }
-//    }
-//  }
-
-}