**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.

log.push('mount ' + this.props.name);
      }
      componentWillUnmount() {
        log.push('unmount ' + this.props.name);
      }
    }
    var renderer = ReactTestRenderer.create(<Log key="foo" name="Foo" />);
    renderer.update(<Log key="bar" name="Bar" />);
    renderer.unmount();
    expect(log).toEqual([
      'render Foo',
      'mount Foo',
      'unmount Foo',
      'render Bar',
      'mount Bar',
      'unmount Bar',
    ]);
  });
  it('gives a ref to native components', () => {
    var log = [];
    ReactTestRenderer.create(<div ref={(r) => log.push(r)} />);
    expect(log).toEqual([null]);
  });
  it('warns correctly for refs on SFCs', () => {
    spyOn(console, 'error');
    function Bar() {
      return <div>Hello, world</div>;
    }
    class Foo extends React.Component {