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".
itRenders('no className prop with null value', async render => {
        const e = await render(<div className={null} />);
        expect(e.hasAttribute('className')).toBe(false);
      });
      itRenders('badly cased className with a warning', async render => {
        const e = await render(<div classname="test" />, 1);
        expect(e.hasAttribute('class')).toBe(false);
        expect(e.hasAttribute('classname')).toBe(true);
      });
      itRenders(
        'className prop when given the alias with a warning',
        async render => {
          const e = await render(<div class="test" />, 1);
          expect(e.className).toBe('test');
        },
      );
      itRenders(
        'className prop when given a badly cased alias',
        async render => {
          const e = await render(<div cLASs="test" />, 1);
          expect(e.className).toBe('test');
        },
      );
      itRenders('class for custom elements', async render => {
        const e = await render(<div is="custom-element" class="test" />, 0);
        expect(e.getAttribute('class')).toBe('test');
      });
      itRenders('className for custom elements', async render => {
        const e = await render(<div is="custom-element" className="test" />, 0);
        expect(e.getAttribute('className')).toBe('test');