<s>I'm going to revert</s> I have reverted https://github.com/facebook/react/pull/5947 because it's [very confusing](https://mobile.twitter.com/ReactStudent/status/1018749513572368384) to see Proxies in the console. The warning is just not worth the debugging impediment.

In the future, we should consider sealing or preventing extensions to pooled events (if we're not getting rid of pooling altogether, that is). But that would be a breaking change.
`you're seeing this, you're accessing the property \`${property}\` 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.';
    
    expect(() => expect(syntheticEvent.type).toBe(null)).toWarnDev(
      getExpectedWarning('type'),
      {withoutStack: true},
    );
    expect(() => expect(syntheticEvent.nativeEvent).toBe(null)).toWarnDev(
      getExpectedWarning('nativeEvent'),
      {withoutStack: true},
    );
    expect(() => expect(syntheticEvent.target).toBe(null)).toWarnDev(
      getExpectedWarning('target'),
      {withoutStack: true},
    );
    expect(expectedCount).toBe(1);
  });
  it('should warn when setting properties of a synthetic event that has not been persisted', () => {
    let node;
    let expectedCount = 0;
    let syntheticEvent;
    const eventHandler = e => {
      syntheticEvent = e;
      expectedCount++;
    };
    node = ReactDOM.render(<div onClick={eventHandler} />, container);
    const event = document.createEvent('Event');
    event.initEvent('click', true, true);