**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)
render() {
        return <span />;
      }
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(<SomeComponent />);
    expect(shallowRenderer.getMountedInstance().state).toBeNull();
  });
  it('should invoke both deprecated and new lifecycles if both are present', () => {
    const log = [];
    class Component extends React.Component {
      componentWillMount() {
        log.push('componentWillMount');
      }
      componentWillReceiveProps() {
        log.push('componentWillReceiveProps');
      }
      componentWillUpdate() {
        log.push('componentWillUpdate');
      }
      UNSAFE_componentWillMount() {
        log.push('UNSAFE_componentWillMount');
      }
      UNSAFE_componentWillReceiveProps() {
        log.push('UNSAFE_componentWillReceiveProps');
      }
      UNSAFE_componentWillUpdate() {