anchor.diff:    ModalAlert(String title, String message, {Callback accept})
         : super(title, message) {
       modalWindow.addClass('alert');
  -    addButton('Accept', accept);
  +    final btn = addButton('Accept', action: accept);
       modalWindow.open();
  +    btn.nativeElement.focus();
     }
   }
   
  
positive.code:   ModalConfirm(String title, String message, {Callback accept, Callback cancel})
        : super(title, message, accept: accept) {
      modalWindow.addClass('confirm');
      addButton('Cancel', cancel);
      modalWindow.open();
    }
  }
  
negative.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);
  