**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.

* control data that is not handled by React's data flow.
 *
 * @class ReactOwner
 */
var ReactOwner = {
  /**
   * Adds a component by ref to an owner component.
   *
   * @param {ReactComponent} component Component to reference.
   * @param {string} ref Name by which to refer to the component.
   * @param {ReactOwner} owner Component on which to record the ref.
   * @final
   * @internal
   */
  addComponentAsRefTo: function(
    component: ReactInstance,
    ref: string,
    owner: ReactInstance,
    transaction: Transaction,
  ): void {
    invariant(
      isValidOwner(owner),
      'addComponentAsRefTo(...): Only a ReactOwner can have refs. You might ' +
      'be adding a ref to a component that was not created inside a component\'s ' +
      '`render` method, or you have multiple copies of React loaded ' +
      '(details: https://fb.me/react-refs-must-have-owner).'
    );
    owner.attachRef(ref, component, transaction);
  },
  /**