**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.
return <div>{`${this.props.foo}:${this.state.bar}`}</div>;
      }
    }
    const shallowRenderer = createRenderer();
    let result = shallowRenderer.render(<SimpleComponent foo="foo" />);
    expect(result).toEqual(<div>foo:bar</div>);
    const instance = shallowRenderer.getMountedInstance();
    const cloned = React.cloneElement(instance, {foo: 'baz'});
    result = shallowRenderer.render(cloned);
    expect(result).toEqual(<div>baz:bar</div>);
  });
  describe('Simulate', () => {
    it('should set the type of the event', () => {
      let event;
      const stub = jest.genMockFn().mockImplementation(e => {
        e.persist();
        event = e;
      });
      const container = document.createElement('div');
      const instance = ReactDOM.render(<div onKeyDown={stub} />, container);
      const node = ReactDOM.findDOMNode(instance);
      ReactTestUtils.Simulate.keyDown(node);
      expect(event.type).toBe('keydown');
      expect(event.nativeEvent.type).toBe('keydown');
    });
    it('should work with renderIntoDocument', () => {
      const onChange = jest.fn();
      class MyComponent extends React.Component {