<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.
it('should warn if Proxy is supported and the synthetic event is added a property', () => {
    let node;
    let expectedCount = 0;
    let syntheticEvent;
    const eventHandler = e => {
      if (typeof Proxy === 'function') {
        expect(() => {
          e.foo = 'bar';
        }).toWarnDev(
          'Warning: This synthetic event is reused for performance reasons. If ' +
            "you're seeing this, you're adding a new property in the synthetic " +
            'event object. The property is never released. ' +
            'See https://fb.me/react-event-pooling for more information.',
          {withoutStack: true},
        );
      } else {
        e.foo = 'bar';
      }
      syntheticEvent = e;
      expectedCount++;
    };
    node = ReactDOM.render(<div onClick={eventHandler} />, container);
    const event = document.createEvent('Event');
    event.initEvent('click', true, true);
    node.dispatchEvent(event);
    expect(syntheticEvent.foo).toBe('bar');
    expect(expectedCount).toBe(1);
  });
});