Observed this on react@16.0.0-beta.5

```js
import React from 'react';
import { renderToString } from 'react-dom/server';

const B = () => 'b';

console.log(renderToString(<div>{'a'}<B />{'c'}</div>));
```

This renders as `<div data-reactroot="">a<!-- -->bc</div>`. If you try to hydrate this you'll get the error: Text content did not match. Server: "bc" Client: "b".
/**
 * 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 ExecutionEnvironment;
let PropTypes;
let React;
let ReactDOM;
let ReactDOMServer;
let ReactTestUtils;
const stream = require('stream');
const TEXT_NODE_TYPE = 3;
function asyncReactDOMRender(reactElement, domElement, forceHydrate) {
  return new Promise(resolve => {
    if (forceHydrate) {
      ReactDOM.hydrate(reactElement, domElement);
    } else {
      ReactDOM.render(reactElement, domElement);
    }
    
    
    resolve();
  });
}
async function expectErrors(fn, count) {