**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)
return {
    memoizedState: null,
    queue: null,
    next: null,
  };
}
function basicStateReducer<S>(state: S, action: BasicStateAction<S>): S {
  return typeof action === 'function' ? action(state) : action;
}
class ReactShallowRenderer {
  static createRenderer = function() {
    return new ReactShallowRenderer();
  };
  constructor() {
    this._context = null;
    this._element = null;
    this._instance = null;
    this._newState = null;
    this._rendered = null;
    this._rendering = false;
    this._forcedUpdate = false;