**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.
var toggleActivatedState = this._toggleActivatedState;
        return !this.state.activated
          ? <a ref="x" onClick={toggleActivatedState} />
          : <b ref="x" onClick={toggleActivatedState} />;
      }
    };
    /**
     * We'll use this to ensure that an old version is not cached when it is
     * reallocated again.
     */
    ChildUpdates = class extends React.Component {
      getAnchor = () => {
        return this.refs.anch;
      };
      render() {
        var className = this.props.anchorClassOn ? 'anchorClass' : '';
        return this.props.renderAnchor
          ? <a ref="anch" className={className} />
          : <b />;
      }
    };
  });
  it('should support module pattern components', () => {
    function Child({test}) {