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.
/**
 * Copyright (c) 2013-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @emails react-core
 */
'use strict';
let React;
let ReactDOM;
describe('SyntheticEvent', () => {
  let container;
  beforeEach(() => {
    React = require('react');
    ReactDOM = require('react-dom');
    container = document.createElement('div');
    document.body.appendChild(container);
  });
  afterEach(() => {
    document.body.removeChild(container);
    container = null;
  });
  it('should be able to `preventDefault`', () => {
    let node;
    let expectedCount = 0;
    const eventHandler = syntheticEvent => {