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.
return true;
}
function functionThatReturnsFalse() {
  return false;
}
/**
 * Synthetic events are dispatched by event plugins, typically in response to a
 * top-level event delegation handler.
 *
 * These systems should generally use pooling to reduce the frequency of garbage
 * collection. The system should check `isPersistent` to determine whether the
 * event should be released into the pool after being dispatched. Users that
 * need a persisted event should invoke `persist`.
 *
 * Synthetic events (and subclasses) implement the DOM Level 3 Events API by
 * normalizing browser quirks. Subclasses do not necessarily have to implement a
 * DOM interface; custom application-specific events can also subclass this.
 *
 * @param {object} dispatchConfig Configuration used to dispatch this event.
 * @param {*} targetInst Marker identifying the event target.
 * @param {object} nativeEvent Native browser event.
 * @param {DOMEventTarget} nativeEventTarget Target node.
 */
function SyntheticEvent(