**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 (
          <button ref="button" onClick={this.onClick}>
            {this.state.counter}
          </button>
        );
      }
    }
    const shallowRenderer = createRenderer();
    let result = shallowRenderer.render(<SimpleComponent defaultCount={1} />);
    expect(result.props.children).toEqual(0);
    instance.setState((state, props) => {
      return {counter: props.defaultCount + 1};
    });
    result = shallowRenderer.getRenderOutput();
    expect(result.props.children).toEqual(2);
  });
  it('can access component instance from setState updater function', done => {
    let instance;
    class SimpleComponent extends React.Component {