anchor.diff:    ModalConfirm(String title, String message, {Callback accept, Callback cancel})
         : super(title, message, accept: accept) {
       modalWindow.addClass('confirm');
  -    addButton('Cancel', cancel);
  +    addButton('Cancel', action: cancel);
       modalWindow.open();
     }
   }
  
positive.code:   /// Adds a button with a [label].
    ///
    /// When the user presses the button, it calls the [action] function.
    DomElement addButton(String label, [Callback action]) {
      final btn = $('<button type="button" />')
        ..text = label
        ..addTo(_footer.element);
  
negative.code:   ModalAlert(String title, String message, {Callback accept})
        : super(title, message) {
      modalWindow.addClass('alert');
      final btn = addButton('Accept', action: accept);
      modalWindow.open();
      btn.nativeElement.focus();
    }
  }
  
  