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.
 *
 * @providesModule ReactPartialRenderer
 */
'use strict';
var {
  Namespaces,
  getIntrinsicNamespace,
  getChildNamespace,
} = require('DOMNamespaces');
var DOMMarkupOperations = require('DOMMarkupOperations');
var React = require('react');
var ReactControlledValuePropTypes = require('ReactControlledValuePropTypes');
var assertValidProps = require('assertValidProps');
var dangerousStyleValue = require('dangerousStyleValue');
var emptyObject = require('fbjs/lib/emptyObject');
var escapeTextContentForBrowser = require('escapeTextContentForBrowser');
var hyphenateStyleName = require('fbjs/lib/hyphenateStyleName');
var invariant = require('fbjs/lib/invariant');
var memoizeStringOnly = require('fbjs/lib/memoizeStringOnly');
var omittedCloseTags = require('omittedCloseTags');
var isCustomComponent = require('isCustomComponent');