As initially reported in https://github.com/facebook/react/issues/11520#issuecomment-343593640, attempting to upgrade from v16.0 to v16.1 caused a test that was previously working to fail. The test was using [sinon](http://sinonjs.org/) to stub `renderToStaticMarkup` on `react-dom/server` and v16.1 results in the following error: 

```
TypeError: Cannot redefine property: renderToStaticMarkup
    at Function.defineProperty (<anonymous>)
    at wrapMethod (node_modules/sinon/lib/sinon/util/core/wrap-method.js:92:16)
    at stub (node_modules/sinon/lib/sinon/stub.js:60:44)
    at Object.stub (node_modules/sinon/lib/sinon/collection.js:102:33)
    at Context.setup (test/unit/....)
```

the export from `react-dom/cjs/react-dom-server.node.development` is frozen as:

```js
Object.freeze({
  renderToString: renderToString,
  renderToStaticMarkup: renderToStaticMarkup,
  renderToNodeStream: renderToNodeStream,
  renderToStaticNodeStream: renderToStaticNodeStream,
  version: version
});
```

[the source](https://github.com/facebook/react/blob/13c491a8282cb102c40d4ed74738ebebe7901a5f/packages/react-dom/src/server/ReactDOMServerNode.js#L17-L22) is not frozen, and https://github.com/facebook/react/issues/11520#issuecomment-343604002 further confirms that the `Object.freeze()` is a result of the transpilation.

for now, i am simply splitting this report into a separate issue from https://github.com/facebook/react/issues/11520, but i will add a reproduction soon and plan to submit a PR as a follow up.


/**
 * 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.
 *
 * @flow
 */
'use strict';
module.exports = require('./src/server/ReactDOMServerBrowser');