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.
event = e;
    }
    const node = ReactDOM.render(<div onClick={assignEvent} />, container);
    node.click();
    
    expect(() => {
      event.nativeEvent; 
    }).toWarnDev(
      'Warning: This synthetic event is reused for performance reasons. If ' +
        "you're seeing this, you're accessing the property `nativeEvent` 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.',
      {withoutStack: true},
    );
  });