**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.
<ChildUpdates renderAnchor={false} anchorClassOn={true} />,
      container,
    );
    ReactDOM.render(
      
      <ChildUpdates renderAnchor={true} anchorClassOn={false} />,
      container,
    );
    expect(instance.getAnchor().className).toBe('');
  });
  it('should use default values for undefined props', () => {
    class Component extends React.Component {
      static defaultProps = {prop: 'testKey'};
      render() {
        return <span />;
      }
    }
    var instance1 = ReactTestUtils.renderIntoDocument(<Component />);
    expect(instance1.props).toEqual({prop: 'testKey'});
    var instance2 = ReactTestUtils.renderIntoDocument(
      <Component prop={undefined} />,
    );
    expect(instance2.props).toEqual({prop: 'testKey'});
    var instance3 = ReactTestUtils.renderIntoDocument(
      <Component prop={null} />,
    );
    expect(instance3.props).toEqual({prop: null});
  });
  it('should not mutate passed-in props object', () => {
    class Component extends React.Component {