To reproduce, open http://jsfiddle.net/0jjnaghw/
Error is logged to browser dev tools (in Chrome & Firefox, somehow not in Safari).

Happens on v15.0.0-rc.2 and on latest master version.

The assignment of additional event data to the newly created SyntheticEvent triggers a console error:
https://github.com/facebook/react/blob/3a4e1dbb5b2957875cab6af11a5e5934e125a0d8/src/test/ReactTestUtils.js#L502

> "Warning: This synthetic event is reused for performance reasons."

).toBe(true);
    expect(createEvent({returnValue: false}).isDefaultPrevented()).toBe(true);
  });
  it('should be able to `stopPropagation`', function() {
    var nativeEvent = {};
    var syntheticEvent = createEvent(nativeEvent);
    expect(syntheticEvent.isPropagationStopped()).toBe(false);
    syntheticEvent.stopPropagation();
    expect(syntheticEvent.isPropagationStopped()).toBe(true);
    expect(nativeEvent.cancelBubble).toBe(true);
  });
  it('should be able to `persist`', function() {
    var syntheticEvent = createEvent({});
    expect(syntheticEvent.isPersistent()).toBe(false);
    syntheticEvent.persist();
    expect(syntheticEvent.isPersistent()).toBe(true);
  });
  it('should be nullified if the synthetic event has called destructor and log warnings', function() {
    spyOn(console, 'error');
    var target = document.createElement('div');
    var syntheticEvent = createEvent({srcElement: target});
    syntheticEvent.destructor();
    expect(syntheticEvent.type).toBe(null);
    expect(syntheticEvent.nativeEvent).toBe(null);
    expect(syntheticEvent.target).toBe(null);
    
    expect(console.error.calls.length).toBe(3);
    
    expect(console.error.argsForCall[0][0]).toBe(
      'Warning: This synthetic event is reused for performance reasons. If ' +