anchor.diff:  
   @JS('React.addons.TestUtils.Simulate')
   external dynamic get _Simulate;
  -
   
   void _typedCallbackRefTests<T>(react.ReactComponentFactoryProxy factory) {
     if (T == dynamic) {
  
positive.code:     });
  
      group('passing Dart events to Dart handlers, and JS events to handlers originating from JS:', () {
        for (var helper in helpers) {
          test(helper.description, () {
            helper.simulate(node);
            expect(events[helper], isNotNull, reason: 'handler should have been called');
            expect(events[helper], helper.isDart ? isA<react.SyntheticMouseEvent>() : isNot(isA<react.SyntheticMouseEvent>()));
          });
        }
      });
  
negative.code:   group('forwardRef wraps event handlers properly,', () {
      final isDartComponent = factory is ReactDartComponentFactoryProxy;
  
      const dartInside = EventTestCase.dart('onMouseDown', 'inside forwardRef');
      const dart = EventTestCase.dart('onMouseUp', 'set on forwardRef hoc');
      const dartCloned = EventTestCase.dart('onMouseLeave', 'cloned onto forwardRef hoc');
      const jsCloned = EventTestCase.js('onClick', 'cloned onto forwardRef hoc ');
      const eventCases = {dartInside, dart, jsCloned, dartCloned};
  
      Element node;
      Map<EventTestCase, dynamic> events;
      Map propsFromDartRender;
  
      setUpAll(() {
        expect(eventCases.map((h) => h.eventPropKey).toSet(), hasLength(eventCases.length),
            reason: 'test setup: each helper should have a unique event key');
  
        node = null;
  