**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.

*
   * @final
   * @internal
   */
  unmountComponent: function(safely, skipLifecycle) {
    if (!this._renderedComponent) {
      return;
    }
    var inst = this._instance;
    if (inst.componentWillUnmount && !inst._calledComponentWillUnmount) {
      inst._calledComponentWillUnmount = true;
      if (safely) {
        if (!skipLifecycle) {
          var name = this.getName() + '.componentWillUnmount()';
          ReactErrorUtils.invokeGuardedCallback(name, inst.componentWillUnmount.bind(inst));
        }
      } else {
        if (__DEV__) {
          measureLifeCyclePerf(
            () => inst.componentWillUnmount(),
            this._debugID,
            'componentWillUnmount'
          );
        } else {
          inst.componentWillUnmount();
        }
      }
    }
    if (this._renderedComponent) {
      ReactReconciler.unmountComponent(