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.
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() {},