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".
tag === element.type,
          '<%s /> is using uppercase HTML. Always use lowercase HTML tags ' +
            'in React.',
          element.type,
        );
      }
    }
    validateDangerousTag(tag);
    var props = element.props;
    if (tag === 'input') {
      if (__DEV__) {
        ReactControlledValuePropTypes.checkPropTypes(
          'input',
          props,
          () => '', 
        );
        if (
          props.checked !== undefined &&
          props.defaultChecked !== undefined &&
          !didWarnDefaultChecked
        ) {
          warning(
            false,
            '%s contains an input of type %s with both checked and defaultChecked props. ' +
              'Input elements must be either controlled or uncontrolled ' +
              '(specify either the checked prop, or the defaultChecked prop, but not ' +
              'both). Decide between using a controlled or uncontrolled input ' +
              'element and remove one of these props. More info: ' +
              'https://fb.me/react-controlled-components',
            'A component',