<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.
!target.constructor.Interface.hasOwnProperty(prop) &&
              shouldBeReleasedProperties.indexOf(prop) === -1
            ) {
              warningWithoutStack(
                didWarnForAddedNewProperty || target.isPersistent(),
                "This synthetic event is reused for performance reasons. If you're " +
                  "seeing this, you're adding a new property in the synthetic event object. " +
                  'The property is never released. See ' +
                  'https://fb.me/react-event-pooling for more information.',
              );
              didWarnForAddedNewProperty = true;
            }
            target[prop] = value;
            return true;
          },
        });
      },
    });
    
  }
}
addEventPoolingTo(SyntheticEvent);
/**
 * Helper to nullify syntheticEvent instance properties when destructing
 *
 * @param {String} propName
 * @param {?object} getVal
 * @return {object} defineProperty object
 */
function getPooledWarningPropertyDefinition(propName, getVal) {