Currently, if you define `shouldComponentUpdate` on `React.PureComponent` nothing bad happens and it acts like `React.Component` https://jsfiddle.net/jekmdfva/

We should probably warn in dev since it defaults the point of `PureComponent`
/**
 * Copyright 2013-present, Facebook, Inc.
 * All rights reserved.
 *
 * This source code is licensed under the BSD-style license found in the
 * LICENSE file in the root directory of this source tree. An additional grant
 * of patent rights can be found in the PATENTS file in the same directory.
 *
 * @providesModule ReactFiberClassComponent
 * @flow
 */
'use strict';
import type {Fiber} from 'ReactFiber';
import type {PriorityLevel} from 'ReactPriorityLevel';
var {Update} = require('ReactTypeOfSideEffect');
var {
  cacheContext,
  getMaskedContext,
  getUnmaskedContext,
  isContextConsumer,
} = require('ReactFiberContext');
var {
  addUpdate,
  addReplaceUpdate,
  addForceUpdate,
  beginUpdateQueue,
} = require('ReactFiberUpdateQueue');
var {hasContextChanged} = require('ReactFiberContext');