anchor.diff:  import 'package:js/js.dart';
   
   @JS()
  -@anonymous
   class SyntheticEvent {
     external bool get bubbles;
     external bool get cancelable;
  
positive.code: }
  
  @JS()
  @anonymous
  class SyntheticPointerEvent extends SyntheticEvent {
    external num get pointerId;
    external num get width;
  
negative.code: }
  
  @JS()
  class SyntheticMouseEvent extends SyntheticEvent {
    external bool get altKey;
    external num get button;
  