anchor.diff:   * http://learningwebgl.com/blog/?p=571
    * https://github.com/martinsik/dart-webgl-tutorials
    */
  -class Lesson07 {
  +class TwoBarrels {
   
     CanvasElement _canvas;
     webgl.RenderingContext _gl;
  
positive.code: }
  
  void main() {
    Lesson07 lesson = new Lesson07(document.querySelector('#game'));
    lesson.start();
  }
  
negative.code:     _gl.uniformMatrix3fv(_uIdentMatrix, false, identMatrix.storage);
    }
  
    void render(double time) {
      _gl.viewport(0, 0, _viewportWidth, _viewportHeight);
      _gl.clear(webgl.RenderingContext.COLOR_BUFFER_BIT | webgl.RenderingContext.DEPTH_BUFFER_BIT);
  
  