**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)
if (nextProps.value === prevState.value) {
          return null;
        }
        return {value: nextProps.value};
      }
      shouldComponentUpdate(nextProps, nextState) {
        return nextState.value !== this.state.value;
      }
      render() {
        return <div>{`value:${this.state.value}`}</div>;
      }
    }
    const shallowRenderer = createRenderer();
    const initialResult = shallowRenderer.render(
      <SimpleComponent value="initial" />,
    );
    expect(initialResult).toEqual(<div>value:initial</div>);
    const updatedResult = shallowRenderer.render(
      <SimpleComponent value="updated" />,
    );
    expect(updatedResult).toEqual(<div>value:updated</div>);
  });
  it('can setState with an updater function', () => {
    let instance;
    class SimpleComponent extends React.Component {