**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.

var ReactMultiChild = require('ReactMultiChild');
var ReactHostComponent = require('ReactHostComponent');
var ReactTestMount = require('ReactTestMount');
var ReactTestReconcileTransaction = require('ReactTestReconcileTransaction');
var ReactUpdates = require('ReactUpdates');
var ReactTestTextComponent = require('ReactTestTextComponent');
var ReactTestEmptyComponent = require('ReactTestEmptyComponent');
import type { ReactElement } from 'ReactElementType';
import type { ReactInstance } from 'ReactInstanceType';
type ReactTestRendererJSON = {
  type: string,
  props: { [propName: string]: string },
  children: null | Array<string | ReactTestRendererJSON>,
  $$typeof?: any
}
/**
 * Drill down (through composites and empty components) until we get a native or
 * native text component.
 *
 * This is pretty polymorphic but unavoidable with the current structure we have
 * for `_renderedChildren`.
 */
function getRenderedHostOrTextFromComponent(component) {
  var rendered;
  while ((rendered = component._renderedComponent)) {
    component = rendered;
  }
  return component;
}
class ReactTestComponent {