I think maybe we should revert https://github.com/facebook/react/pull/5947.

People already think `proxyEvent` is some kind of an API: https://github.com/facebook/react/issues/12169.

It's also annoying to view in the debugger because none of the properties show up.

Instead, we could seal the event object or something like that.
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);
  });
});