**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)
/**
 * Copyright (c) Facebook, Inc. and its affiliates.
 *
 * This source code is licensed under the MIT license found in the
 * LICENSE file in the root directory of this source tree.
 *
 * @flow
 */
import React from 'react';
import {isForwardRef} from 'react-is';
import describeComponentFrame from 'shared/describeComponentFrame';
import getComponentName from 'shared/getComponentName';
import shallowEqual from 'shared/shallowEqual';
import invariant from 'shared/invariant';
import checkPropTypes from 'prop-types/checkPropTypes';
import ReactSharedInternals from 'shared/ReactSharedInternals';
import warning from 'shared/warning';
import is from 'shared/objectIs';
import type {Dispatcher as DispatcherType} from 'react-reconciler/src/ReactFiberHooks';
import type {ReactContext} from 'shared/ReactTypes';