**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)
log.push('render');
        instance = this;
        return null;
      }
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(<Component />);
    log.length = 0;
    instance.setState(() => null);
    instance.setState(() => undefined);
    instance.setState(null);
    instance.setState(undefined);
    expect(log).toEqual([]);
    instance.setState(state => ({count: state.count + 1}));
    expect(log).toEqual(['render']);
  });
  it('should not get this in a function component', () => {
    const logs = [];
    function Foo() {
      logs.push(this);
      return <div>foo</div>;
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(<Foo foo="bar" />);
    expect(logs).toEqual([undefined]);
  });
});