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".
var toArray = React.Children.toArray;
if (__DEV__) {
  var warning = require('fbjs/lib/warning');
  var checkPropTypes = require('prop-types/checkPropTypes');
  var warnValidStyle = require('warnValidStyle');
  var {
    validateProperties: validateARIAProperties,
  } = require('ReactDOMInvalidARIAHook');
  var {
    validateProperties: validateInputProperties,
  } = require('ReactDOMNullInputValuePropHook');
  var {
    validateProperties: validateUnknownProperties,
  } = require('ReactDOMUnknownPropertyHook');
  var validatePropertiesInDevelopment = function(type, props) {
    validateARIAProperties(type, props);
    validateInputProperties(type, props);
    validateUnknownProperties(type, props);
  };
  var describeComponentFrame = require('describeComponentFrame');
  var describeStackFrame = function(element): string {
    var source = element._source;
    var type = element.type;
    var name = getComponentName(type);
    var ownerName = null;
    return describeComponentFrame(name, source, ownerName);
  };
  var {ReactDebugCurrentFrame} = require('ReactGlobalSharedState');
  var currentDebugStack = null;
  var currentDebugElementStack = null;