**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 element = this._currentElement;
    var options = transaction.getTestOptions();
    return options.createNodeMock(element);
  }
  toJSON(): ReactTestRendererJSON {
    var {children, ...props} = this._currentElement.props;
    var childrenJSON = [];
    for (var key in this._renderedChildren) {
      var inst = this._renderedChildren[key];
      inst = getRenderedHostOrTextFromComponent(inst);
      var json = inst.toJSON();
      if (json !== undefined) {
        childrenJSON.push(json);
      }
    }
    var object: ReactTestRendererJSON = {
      type: this._currentElement.type,
      props: props,
      children: childrenJSON.length ? childrenJSON : null,
    };
    Object.defineProperty(object, '$$typeof', {
      value: Symbol.for('react.test.json'),
    });
    return object;
  }
  getHostNode(): void {}