code: function (duration, callback) {
              this._flicker.duration = duration;
              if (this._flicker.duration <= 0) {
                  this._flicker.isFlickering = false;
                  this._flicker.callback = null;
              }
              else if (!this._flicker.isFlickering) {
                  this._flicker.callback = callback;
                  this._flicker.isFlickering = true;
              }
              return this;
          }
docs: make the object flicker
  @name flicker
  @memberOf me.Sprite.prototype
  @function
  @param {Number} duration expressed in milliseconds
  @param {Function} callback Function to call when flickering ends
  @return {me.Sprite} Reference to this object for method chaining
  @example
  // make the object flicker for 1 second
  // and then remove it
  this.flicker(1000, function () {
  me.game.world.removeChild(this);
  });