**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.
state = {value: 0};
      render() {
        renderPasses++;
        renderedState = this.state.value;
        if (this.state.value === 0) {
          this.setState({value: 1});
        }
        return <div />;
      }
    }
    expectDev(console.error.calls.count()).toBe(0);
    var instance = ReactDOM.render(<Component />, container);
    expectDev(console.error.calls.count()).toBe(1);
    expectDev(console.error.calls.argsFor(0)[0]).toContain(
      'Cannot update during an existing state transition (such as within ' +
        "`render` or another component's constructor). Render methods should " +
        'be a pure function of props and state; constructor side-effects are ' +
        'an anti-pattern, but can be moved to `componentWillMount`.',
    );
    
    
    
    expect(renderPasses).toBe(2);
    expect(renderedState).toBe(1);
    expect(instance.state.value).toBe(1);
    
    var instance2 = ReactDOM.render(<Component prop={123} />, container);
    expect(instance).toBe(instance2);
    expect(renderedState).toBe(1);