**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)
constructor(props, context) {
        super(props, context);
        this.state = initialState;
      }
      static contextTypes = {
        context: PropTypes.string,
      };
      componentDidUpdate(...args) {
        componentDidUpdateParams.push(...args);
      }
      static getDerivedStateFromProps(...args) {
        getDerivedStateFromPropsParams.push(args);
        return null;
      }
      shouldComponentUpdate(...args) {
        shouldComponentUpdateParams.push(...args);
        return true;
      }
      render() {
        return null;
      }
    }