anchor.diff:  
     bool get isReadonly => (readonly??false);
   
  +}
  +
  +class BeforeFormSubmitEvent {
  +
   }
positive.code:   /// Only propagates the submit event when this form is valid.
    /// Forces validation of all components inside this form.
    void submitForm(Event event) {
      if (event != null) {
        event.preventDefault();
        event.stopPropagation();
  
negative.code:   @Output()
    final EventEmitter<Event> submit = new EventEmitter<Event>();
  
    @Output()
    final EventEmitter<BeforeFormSubmitEvent> beforeSubmit = new EventEmitter<BeforeFormSubmitEvent>(false);
  
    String id = ui.uid('form-');
  
    List<formValidatorFunction> _validators = [];
  