**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(renderer) {
    this._renderer = renderer;
    this._callbacks = [];
  }
  _renderer: ReactShallowRenderer;
  _callbacks: Array<any>;
  _enqueueCallback(callback, publicInstance) {
    if (typeof callback === 'function' && publicInstance) {
      this._callbacks.push({
        callback,
        publicInstance,
      });
    }
  }
  _invokeCallbacks() {
    const callbacks = this._callbacks;
    this._callbacks = [];
    callbacks.forEach(({callback, publicInstance}) => {
      callback.call(publicInstance);
    });
  }
  isMounted(publicInstance) {
    return !!this._renderer._element;
  }
  enqueueForceUpdate(publicInstance, callback, callerName) {
    this._enqueueCallback(callback, publicInstance);
    this._renderer._forcedUpdate = true;
    this._renderer.render(this._renderer._element, this._renderer._context);
  }
  enqueueReplaceState(publicInstance, completeState, callback, callerName) {