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.