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

**What is the current behavior?**
When I call setState with a callback in a test using react shallow renderer (via enzyme), the callback gets called and `this.state` is still the old state.

EDIT: This seems limited to componentWillMount

**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 for React 16: https://jsfiddle.net/Luktwrdm/, template for React 15: https://jsfiddle.net/hmbg7e9w/).**

EDIT: Reproduce with this https://github.com/bdwain/setstate-callback-bug

**What is the expected behavior?**
When the setState callback gets called, it should have access to the new state.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
Version 16.
It worked in v15 with enzyme 2.

EDIT: I think this is because in componentWillMount, [this line](https://github.com/facebook/react/blob/master/packages/react-test-renderer/src/ReactShallowRenderer.js#L67) causes the render method in the shallow wrapper to return early, before it updates the state.
/**
 * Copyright (c) 2013-present, Facebook, Inc.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @emails react-core
 */
'use strict';
let createRenderer;
let PropTypes;
let React;
describe('ReactShallowRenderer', () => {
  beforeEach(() => {
    createRenderer = require('react-test-renderer/shallow').createRenderer;
    PropTypes = require('prop-types');
    React = require('react');
  });
  it('should call all of the lifecycle hooks', () => {
    const logs = [];
    const logger = message => () => logs.push(message) || true;
    class SomeComponent extends React.Component {
      componentWillMount = logger('componentWillMount');
      componentDidMount = logger('componentDidMount');
      componentWillReceiveProps = logger('componentWillReceiveProps');
      shouldComponentUpdate = logger('shouldComponentUpdate');
      componentWillUpdate = logger('componentWillUpdate');
      componentDidUpdate = logger('componentDidUpdate');
      componentWillUnmount = logger('componentWillUnmount');
      render() {