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