anchor.diff:    @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 = [];
  
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: 
    bool get isReadonly => (readonly??false);
  
  }
  
  class BeforeFormSubmitEvent {
  
  }