<s>I'm going to revert</s> I have reverted https://github.com/facebook/react/pull/5947 because it's [very confusing](https://mobile.twitter.com/ReactStudent/status/1018749513572368384) to see Proxies in the console. The warning is just not worth the debugging impediment.

In the future, we should consider sealing or preventing extensions to pooled events (if we're not getting rid of pooling altogether, that is). But that would be a breaking change.
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(