Fix bugs found by Dart analyzer

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

 part of game;
 
 const double _steeringThreshold = 0.0;
 const double _steeringMax = 150.0;
 
 // Random generator
 Math.Random _rand = new Math.Random();
 
 const double _gameSizeWidth = 1024.0;
 const double _gameSizeHeight = 1024.0;
 
 const double _shipRadius = 30.0;
 const double _lrgAsteroidRadius = 40.0;
 const double _medAsteroidRadius = 20.0;
 const double _smlAsteroidRadius = 10.0;
 const double _maxAsteroidSpeed = 1.0;
 
 const int _lifeTimeLaser = 50;
 
 const int _numStarsInStarField = 150;
 
 const int _numFramesShieldActive = 60 * 5;
 const int _numFramesShieldFlickers = 60;
 
 class GameDemoWorld extends NodeWithSize {
-  App _app;
-
   // Images
   Image _imgNebula;
 
   SpriteSheet _spriteSheet;
 
   // Inputs
   double _joystickX = 0.0;
   double _joystickY = 0.0;
-  bool _fire;
 
   Node _gameLayer;
 
   Ship _ship;
   Sprite _shield;
   List<Asteroid> _asteroids = [];
   List<Laser> _lasers = [];
   StarField _starField;
   Nebula _nebula;
 
   // Game state
   int _numFrames = 0;
   bool _isGameOver = false;
 
-  GameDemoWorld(this._app, ImageMap images, this._spriteSheet) : super(new Size(_gameSizeWidth, _gameSizeHeight)) {
+  GameDemoWorld(App app, ImageMap images, this._spriteSheet) : super(new Size(_gameSizeWidth, _gameSizeHeight)) {
 
     // Fetch images
     _imgNebula = images["assets/nebula.png"];
 
     _gameLayer = new Node();
     this.addChild(_gameLayer);
 
     // Add some asteroids to the game world
     for (int i = 0; i < 5; i++) {
       addAsteroid(AsteroidSize.large);
@@ skipping 561 matching lines @@
   double dy = a.y - b.y;
   if (dx < 0.0) dx = -dx;
   if (dy < 0.0) dy = -dy;
   if (dx > dy) {
     return dx + dy/2.0;
   }
   else {
     return dy + dx/2.0;
   }
 }