**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.
state = {
        counter: 0,
      };
      render() {
        instance = this;
        return (
          <button ref="button" onClick={this.onClick}>
            {this.state.counter}
          </button>
        );
      }
    }
    const shallowRenderer = createRenderer();
    let result = shallowRenderer.render(<SimpleComponent defaultCount={1} />);
    expect(result.props.children).toEqual(0);
    instance.setState((state, props) => {
      return {counter: props.defaultCount + 1};
    });
    result = shallowRenderer.getRenderOutput();
    expect(result.props.children).toEqual(2);
  });
  it('can setState with a callback', () => {
    let instance;
    class SimpleComponent extends React.Component {