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".
const e = await render(<div><br /></div>);
      expect(e.childNodes.length).toBe(1);
      expect(e.firstChild.tagName).toBe('BR');
    });
    itRenders('a string', async render => {
      let e = await render('Hello');
      expect(e.nodeType).toBe(3);
      expect(e.nodeValue).toMatch('Hello');
    });
    itRenders('a number', async render => {
      let e = await render(42);
      expect(e.nodeType).toBe(3);
      expect(e.nodeValue).toMatch('42');
    });
    itRenders('an array with one child', async render => {
      let e = await render([<div key={1}>text1</div>]);
      let parent = e.parentNode;
      expect(parent.childNodes[0].tagName).toBe('DIV');
    });
    itRenders('an array with several children', async render => {
      let Header = props => {
        return <p>header</p>;
      };
      let Footer = props => {
        return [<h2 key={1}>footer</h2>, <h3 key={2}>about</h3>];
      };
      let e = await render([
        <div key={1}>text1</div>,
        <span key={2}>text2</span>,
        <Header key={3} />,