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.
const event = this.nativeEvent;
    if (!event) {
      return;
    }
    if (event.preventDefault) {
      event.preventDefault();
    } else if (typeof event.returnValue !== 'unknown') {
      event.returnValue = false;
    }
    this.isDefaultPrevented = functionThatReturnsTrue;
  },
  stopPropagation: function() {
    const event = this.nativeEvent;
    if (!event) {
      return;
    }
    if (event.stopPropagation) {
      event.stopPropagation();
    } else if (typeof event.cancelBubble !== 'unknown') {
      
      
      
      
      
      event.cancelBubble = true;
    }
    this.isPropagationStopped = functionThatReturnsTrue;
  },
  /**
   * We release all dispatched `SyntheticEvent`s after each event loop, adding