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

**What is the current behavior?**
In normal rendering, you can memoize a class-based component: https://jsfiddle.net/586ea3cx/

With the shallow renderer, it seems like you can't: https://jsfiddle.net/odj217Lv/1/

This might be blocking https://github.com/airbnb/enzyme/pull/1914, in which tests are failing with `Cannot call a class as a function`. (it's tough to repro stuff with the shallow renderer)
state = {
        separator: '-',
      };
      UNSAFE_componentWillMount() {
        this.setState({groovy: 'doovy'});
        this.setState({doovy: 'groovy'});
      }
      render() {
        const {groovy, doovy, separator} = this.state;
        return <div>{`${groovy}${separator}${doovy}`}</div>;
      }
    }
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<SimpleComponent />);
    expect(result).toEqual(<div>doovy-groovy</div>);
  });
  it('can setState in componentWillMount with an updater function repeatedly when shallow rendering', () => {
    class SimpleComponent extends React.Component {
      state = {
        separator: '-',
      };
      UNSAFE_componentWillMount() {
        this.setState(state => ({groovy: 'doovy'}));
        this.setState(state => ({doovy: state.groovy}));
      }
      render() {