**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 = {};
      render() {
        instance = this;
        return null;
      }
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(<SimpleComponent />);
    instance.setState(function updater(state, props) {
      expect(this).toBe(instance);
      done();
    });
  });
  it('can setState with a callback', () => {
    let instance;
    class SimpleComponent extends React.Component {