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".
render() {
              return 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 function returning an object',
        async render => {
          const ObjectComponent = () => ({x: 123});
          await render(<ObjectComponent />, 1);
        },
        'Objects are not valid as a React child (found: object with keys ' +
          '{x}). If you meant to render a collection of children, use ' +
          'an array instead.',
      );
      itThrowsWhenRendering(
        'a class returning an object',
        async render => {
          class ObjectComponent extends React.Component {