**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.

}
    if (replace && queue.length === 1) {
      return queue[0];
    }
    var nextState = Object.assign({}, replace ? queue[0] : inst.state);
    for (var i = replace ? 1 : 0; i < queue.length; i++) {
      var partial = queue[i];
      Object.assign(
        nextState,
        typeof partial === 'function' ?
          partial.call(inst, nextState, props, context) :
          partial
      );
    }
    return nextState;
  },
  /**
   * Merges new props and state, notifies delegate methods of update and
   * performs update.
   *
   * @param {ReactElement} nextElement Next element
   * @param {object} nextProps Next public object to set as properties.
   * @param {?object} nextState Next object to set as state.
   * @param {?object} nextContext Next public object to set as context.
   * @param {ReactReconcileTransaction} transaction
   * @param {?object} unmaskedContext
   * @private
   */
  _performComponentUpdate: function(
    nextElement,