**Do you want to request a *feature* or report a *bug*?**
bug

**What is the current behavior?**
In React v16, the callback function that is an optional parameter to the `setState` function is called and passed a value of null.  Previously, in React v15, the function was called with a value of undefined.  This change breaks the default parameters feature of JavaScript.

I discovered this behavior while working on a React Native project (hence the example using View & Text).

**If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem via https://jsfiddle.net or similar (template: https://jsfiddle.net/84v837e9/).**

```
import React, {
	Component,
} from 'react';

export default class MyComp extends Component {
	componentDidMount() {
		this.setState({}, (val) => console.log(val))
	}

	render() {
		return (
			<View>
				<Text>{this.props.climbs}</Text>
			</View>
		);
	}
}
```

**What is the expected behavior?**
The callback function will be called with undefined

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
Broken behavior was observed in React 16 alpha 12 and React Native 0.45.1.  

I replicated the issue in React 15.4.1 and React Native 0.40.0 and it was working as expected.
render() {
        return null;
      }
    }
    class Parent extends React.Component {
      render() {
        return <div><Child /></div>;
      }
    }
    var markup = ReactDOMServer.renderToString(<Parent />);
    var container = document.createElement('div');
    container.innerHTML = markup;
    ReactDOM.render(<Parent />, container);
  });
  it('should react to state changes from callbacks', () => {
    var instance = ReactTestUtils.renderIntoDocument(<MorphingComponent />);
    var el = ReactDOM.findDOMNode(instance);
    expect(el.tagName).toBe('A');
    ReactTestUtils.Simulate.click(el);
    el = ReactDOM.findDOMNode(instance);
    expect(el.tagName).toBe('B');
  });
  it('should rewire refs when rendering to different child types', () => {
    var instance = ReactTestUtils.renderIntoDocument(<MorphingComponent />);
    expect(ReactDOM.findDOMNode(instance.refs.x).tagName).toBe('A');