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".
itThrows(
    `when rendering ${desc} with server string render`,
    () => testFn(serverRender),
    partialMessage,
  );
  itThrows(
    `when rendering ${desc} with clean client render`,
    () => testFn(clientCleanRender),
    partialMessage,
  );
  
  
  itThrows(
    `when rendering ${desc} with client render on top of bad server markup`,
    () =>
      testFn((element, warningCount = 0) =>
        clientRenderOnBadMarkup(element, warningCount - 1),
      ),
    partialMessage,
  );
}
async function testMarkupMatch(serverElement, clientElement, shouldMatch) {