Hello,

After updating to React 15.0.1, we are getting the following error:

```
Uncaught TypeError: Cannot read property 'addEventListener' of null
```

![image](https://cloud.githubusercontent.com/assets/4996164/14602577/5bb4e9f2-0571-11e6-9aa1-98de49f50d76.png)

var Y = React.createClass({
      render: function() {
        return (
          <div>
            <Z />
          </div>
        );
      },
    });
    var Z = React.createClass({
      render: function() {
        return <div />;
      },
      componentWillUpdate: function() {
        x.go();
      },
    });
    var x;
    var y;
    x = ReactTestUtils.renderIntoDocument(<X />);
    y = ReactTestUtils.renderIntoDocument(<Y />);
    expect(ReactDOM.findDOMNode(x).textContent).toBe('0');
    y.forceUpdate();
    expect(ReactDOM.findDOMNode(x).textContent).toBe('1');
  });
  it('should queue updates from during mount', function() {
    
    var a;
    var A = React.createClass({
      getInitialState: function() {