**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.

warning(
        publicComponentInstance != null ||
        component._compositeType !== CompositeTypes.StatelessFunctional,
        'Stateless function components cannot be given refs ' +
        '(See ref "%s" in %s created by %s). ' +
        'Attempts to access this ref will fail.',
        ref,
        componentName,
        this.getName()
      );
    }
    var refs = inst.refs === emptyObject ? (inst.refs = {}) : inst.refs;
    refs[ref] = publicComponentInstance;
  },
  /**
   * Detaches a reference name.
   *
   * @param {string} ref Name to dereference.
   * @final
   * @private
   */
  detachRef: function(ref) {
    var refs = this.getPublicInstance().refs;
    delete refs[ref];
  },
  /**
   * Get a text description of the component that can be used to identify it
   * in error messages.
   * @return {string} The name or null.
   * @internal