anchor.diff:    void start() {
       _init().then((_) {
         _lastTimeP = window.performance.now();
  -//      physicsLoop();
  +
  +      var physicsSystem = world.systems.firstWhere((system) => system.group == 1, orElse: () => null);
  +      if (null != physicsSystem) {
  +        physicsLoop();
  +      }
         window.requestAnimationFrame(_firstUpdate);
       });
     }
  
positive.code:   GameBase(String appName, String canvasSelector, int width, int height,
        {this.spriteSheetName: 'assets',
        this.bodyDefsName: 'assets',
        bool webgl: false})
        : canvas = querySelector(canvasSelector),
          helper = new GameHelper(appName),
          webgl = webgl,
  
negative.code:   /// which contains the assets. Usually the game itself.
    GameBase.noAssets(
        String appName, String canvasSelector, int width, int height,
        {bool webgl: false, bool depthTest: true, bool blending: true})
        : this(appName, canvasSelector, width, height,
              spriteSheetName: null, bodyDefsName: null, webgl: webgl, depthTest: depthTest, blending: blending);
  
    GameBase.noCanvas(String appNahme)
        : canvas = null,
  