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."

describe('SyntheticEvent', function() {
  var createEvent;
  beforeEach(function() {
    SyntheticEvent = require('SyntheticEvent');
    React = require('React');
    ReactDOM = require('ReactDOM');
    ReactTestUtils = require('ReactTestUtils');
    createEvent = function(nativeEvent) {
      var target = require('getEventTarget')(nativeEvent);
      return SyntheticEvent.getPooled({}, '', nativeEvent, target);
    };
  });
  it('should normalize `target` from the nativeEvent', function() {
    var target = document.createElement('div');
    var syntheticEvent = createEvent({srcElement: target});
    expect(syntheticEvent.target).toBe(target);
    expect(syntheticEvent.type).toBe(undefined);
  });
  it('should be able to `preventDefault`', function() {
    var nativeEvent = {};
    var syntheticEvent = createEvent(nativeEvent);
    expect(syntheticEvent.isDefaultPrevented()).toBe(false);
    syntheticEvent.preventDefault();
    expect(syntheticEvent.isDefaultPrevented()).toBe(true);
    expect(syntheticEvent.defaultPrevented).toBe(true);
    expect(nativeEvent.returnValue).toBe(false);
  });
  it('should be prevented if nativeEvent is prevented', function() {
    expect(
      createEvent({defaultPrevented: true}).isDefaultPrevented()