**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 = {
        counter: 0,
      };
      render() {
        instance = this;
        return <p>{this.state.counter}</p>;
      }
    }
    const shallowRenderer = createRenderer();
    const result = shallowRenderer.render(<SimpleComponent />);
    expect(result.props.children).toBe(0);
    const callback = jest.fn(function() {
      expect(this).toBe(instance);
    });
    instance.forceUpdate(callback);
    const updated = shallowRenderer.getRenderOutput();
    expect(updated.props.children).toBe(0);
    expect(callback).toHaveBeenCalled();
  });
  it('can pass context when shallowly rendering', () => {
    class SimpleComponent extends React.Component {