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.
const EventConstructor = this;
  invariant(
    event instanceof EventConstructor,
    'Trying to release an event instance  into a pool of a different type.',
  );
  event.destructor();
  if (EventConstructor.eventPool.length < EVENT_POOL_SIZE) {
    EventConstructor.eventPool.push(event);
  }
}
function addEventPoolingTo(EventConstructor) {
  EventConstructor.eventPool = [];
  EventConstructor.getPooled = getPooledEvent;
  EventConstructor.release = releasePooledEvent;
}
export default SyntheticEvent;