**Do you want to request a _feature_ or report a _bug_?**
Bug

**What is the current behavior?**
It's not possible to test component that use `ref` with the `react-test-renderer` utilitiesTesting: the refs are always `null`.

``` javascript
/* @flow */

import React from 'react';

export default class Foo extends React.Component {    
    /* the future refs */
    bar; 

    componentDidMount() {
        console.log(this.bar); // this.bar is null

        this.bar.doThings() // So this fail
    }

    render() {
        return (
            <div ref={(c) => { console.log('ref cb', c); this.bar = c; }}> {/* The callback is call but, `c` is null*/}
                <p>Hello World</p>
            </div>
        );
    }
}
```

``` javascript

import React from 'react';
import renderer from 'react-test-renderer';

it('should have valide ref', () => {
    const foo = renderer.create(<Foo />);

    expect(foo.toJSON()).toMatchSnapshot();
});
```

**What is the expected behavior?**

The ref should be usable.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
- `react@15.3.1`
- `react-dom@15.3.1` 
- `react-test-renderer@15.3.1` 

Only tested with these versions.

render() {
        return <div ref="baz" />;
      }
    }
    ReactTestRenderer.create(<Baz />);
    ReactTestRenderer.create(<Foo />);
    expect(console.error.calls.count()).toBe(1);
    expect(console.error.calls.argsFor(0)[0]).toContain(
      'Stateless function components cannot be given refs ' +
      '(See ref "foo" in Bar created by Foo). ' +
      'Attempts to access this ref will fail.'
    );
  });
  it('allows an optional createNodeMock function', () => {
    var mockDivInstance = { appendChild: () => {} };
    var mockInputInstance = { focus: () => {} };
    var mockListItemInstance = { click: () => {} };
    var mockAnchorInstance = { hover: () => {} };
    var log = [];
    class Foo extends React.Component {
      componentDidMount() {
        log.push(this.refs.bar);
      }
      render() {
        return (
          <a ref="bar">Hello, world</a>
        );
      }
    }
    function createNodeMock(element) {