**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)
this._instance.state,
              partialState,
            );
          }
        }
        if (element.type.hasOwnProperty('contextTypes')) {
          currentlyValidatingElement = element;
          checkPropTypes(
            element.type.contextTypes,
            this._context,
            'context',
            getName(element.type, this._instance),
            getStackAddendum,
          );
          currentlyValidatingElement = null;
        }
        this._mountClassComponent(element, this._context);
      } else {
        const prevDispatcher = ReactCurrentDispatcher.current;
        ReactCurrentDispatcher.current = this._dispatcher;
        this._prepareToUseHooks(element.type);
        try {
          if (isForwardRef(element)) {
            this._rendered = element.type.render(element.props, element.ref);
          } else {
            this._rendered = element.type.call(
              undefined,
              element.props,
              this._context,
            );