**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 = {update: false};
      render() {
        renderCounter++;
        return <div>{`${renderCounter}`}</div>;
      }
    }
    const shallowRenderer = createRenderer();
    shallowRenderer.render(<SimpleComponent />);
    expect(shallowRenderer.getRenderOutput()).toEqual(<div>1</div>);
    const instance = shallowRenderer.getMountedInstance();
    instance.setState({update: false});
    expect(shallowRenderer.getRenderOutput()).toEqual(<div>1</div>);
    instance.setState({update: true});
    expect(shallowRenderer.getRenderOutput()).toEqual(<div>2</div>);
  });
  it('should not run shouldComponentUpdate during forced update', () => {
    let scuCounter = 0;
    class SimpleComponent extends React.Component {
      state = {count: 1};
      shouldComponentUpdate() {
        scuCounter++;
        return false;
      }
      render() {