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".
if (
          render === serverRender ||
          render === clientRenderOnServerString ||
          render === streamRender
        ) {
          expect(e.childNodes.length).toBe(3);
          expectTextNode(e.childNodes[0], '<span>Text1&quot;</span>');
          expectTextNode(e.childNodes[2], '<span>Text2&quot;</span>');
        } else {
          expect(e.childNodes.length).toBe(2);
          expectTextNode(e.childNodes[0], '<span>Text1&quot;</span>');
          expectTextNode(e.childNodes[1], '<span>Text2&quot;</span>');
        }
      });
    });
    describe('components that throw errors', function() {
      itThrowsWhenRendering(
        'a function returning undefined',
        async render => {
          const UndefinedComponent = () => undefined;
          await render(<UndefinedComponent />, 1);
        },
        'UndefinedComponent(...): Nothing was returned from render. ' +
          'This usually means a return statement is missing. Or, to ' +
          'render nothing, return null.',
      );
      itThrowsWhenRendering(
        'a class returning undefined',
        async render => {
          class UndefinedComponent extends React.Component {