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."

'you\'re seeing this, you\'re accessing the property `type` on a ' +
      'released/nullified synthetic event. This is set to null. If you must ' +
      'keep the original synthetic event around, use event.persist(). ' +
      'See https://fb.me/react-event-pooling for more information.'
    );
  });
  it('should warn when setting properties of a destructored synthetic event', function() {
    spyOn(console, 'error');
    var target = document.createElement('div');
    var syntheticEvent = createEvent({srcElement: target});
    syntheticEvent.destructor();
    expect(syntheticEvent.type = 'MouseEvent').toBe('MouseEvent');
    expect(console.error.calls.length).toBe(1);
    expect(console.error.argsForCall[0][0]).toBe(
      'Warning: This synthetic event is reused for performance reasons. If ' +
      'you\'re seeing this, you\'re setting the property `type` on a ' +
      'released/nullified synthetic event. This is effectively a no-op. If you must ' +
      'keep the original synthetic event around, use event.persist(). ' +
      'See https://fb.me/react-event-pooling for more information.'
    );
  });
  it('should warn if the synthetic event has been released when calling `preventDefault`', function() {
    spyOn(console, 'error');
    var syntheticEvent = createEvent({});
    SyntheticEvent.release(syntheticEvent);
    syntheticEvent.preventDefault();
    expect(console.error.calls.length).toBe(1);
    expect(console.error.argsForCall[0][0]).toBe(
      'Warning: This synthetic event is reused for performance reasons. If ' +
      'you\'re seeing this, you\'re accessing the method `preventDefault` on a ' +