**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.

return null;
    }
    var renderer = ReactTestRenderer.create(<Empty />);
    expect(renderer.toJSON()).toEqual(null);
  });
  it('exposes a type flag', () => {
    function Link() {
      return <a role="link" />;
    }
    var renderer = ReactTestRenderer.create(<Link />);
    var object = renderer.toJSON();
    expect(object.$$typeof).toBe(Symbol.for('react.test.json'));
    
    for (var key in object) {
      if (Object.prototype.hasOwnProperty.call(object, key)) {
        expect(key).not.toBe('$$typeof');
      }
    }
  });
  it('renders some basics with an update', () => {
    var renders = 0;
    class Component extends React.Component {