**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 ReactDOM;
var ReactDOMServer;
var ReactCurrentOwner;
var ReactTestUtils;
var PropTypes;
var shallowEqual;
var shallowCompare;
describe('ReactCompositeComponent', () => {
  beforeEach(() => {
    jest.resetModules();
    React = require('react');
    ReactDOM = require('react-dom');
    ReactDOMServer = require('react-dom/server');
    ReactCurrentOwner = require('react')
      .__SECRET_INTERNALS_DO_NOT_USE_OR_YOU_WILL_BE_FIRED.ReactCurrentOwner;
    ReactTestUtils = require('react-dom/test-utils');
    PropTypes = require('prop-types');
    shallowEqual = require('fbjs/lib/shallowEqual');
    shallowCompare = function(instance, nextProps, nextState) {
      return (
        !shallowEqual(instance.props, nextProps) ||
        !shallowEqual(instance.state, nextState)
      );
    };
    MorphingComponent = class extends React.Component {
      state = {activated: false};
      _toggleActivatedState = () => {
        this.setState({activated: !this.state.activated});
      };
      render() {