anchor.diff:  
   void main() {
     prepareGame();
  -
   }
   
   void prepareGame() {
  
positive.code: 
      var div = new DivElement()
                  ..className = 'pexeso-card'
                  ..append(back)
                  ..append(front);
      container.append(div);
  
      return new Card(div, false, id);
    }
  
negative.code:    * Creates the card with DOM given the [backImage] and [frontImage] and
     * appends it to [container].
     */
    factory Card.withDom(Element container, String backImage, String frontImage, id) {
      var back = new ImageElement(src: backImage)
                  ..className = 'back';
      var front = new ImageElement(src: frontImage)
  