<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.
expect(syntheticEvent.isDefaultPrevented()).toBe(false);
      syntheticEvent.preventDefault();
      expect(syntheticEvent.isDefaultPrevented()).toBe(true);
      expect(syntheticEvent.defaultPrevented).toBe(true);
      expectedCount++;
    };
    node = ReactDOM.render(<div onClick={eventHandler} />, container);
    const event = document.createEvent('Event');
    event.initEvent('click', true, true);
    node.dispatchEvent(event);
    expect(expectedCount).toBe(1);
  });
  it('should be prevented if nativeEvent is prevented', () => {
    let node;
    let expectedCount = 0;
    const eventHandler = syntheticEvent => {
      expect(syntheticEvent.isDefaultPrevented()).toBe(true);
      expectedCount++;
    };
    node = ReactDOM.render(<div onClick={eventHandler} />, container);
    let event;
    event = document.createEvent('Event');
    event.initEvent('click', true, true);
    event.preventDefault();
    node.dispatchEvent(event);
    event = document.createEvent('Event');
    event.initEvent('click', true, true);
    
    Object.defineProperty(event, 'defaultPrevented', {
      get() {},