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>{3}</div>);
        expect(e.textContent).toBe('3');
      });
      
      itRenders('zero as single child', async render => {
        const e = await render(<div>{0}</div>);
        expect(e.textContent).toBe('0');
      });
      itRenders('an element with number and text children', async render => {
        const e = await render(<div>{'foo'}{40}</div>);
        
        if (
          render === serverRender ||
          render === clientRenderOnServerString ||
          render === streamRender
        ) {
          
          expect(e.childNodes.length).toBe(3);
          expectTextNode(e.childNodes[0], 'foo');
          expectTextNode(e.childNodes[2], '40');
        } else {
          expect(e.childNodes.length).toBe(2);
          expectTextNode(e.childNodes[0], 'foo');
          expectTextNode(e.childNodes[1], '40');
        }
      });
    });
    describe('null, false, and undefined children', function() {
      itRenders('null single child as blank', async render => {
        const e = await render(<div>{null}</div>);