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**Do you want to request a *feature* or report a *bug*?**
Reporting a bug.

**What is the current behavior?**
With the recent change to `shouldConstruct` (https://github.com/facebook/react/commit/5031ebf6beddf88cac15f4d2c9e91f8dbb91d59d#diff-9ba7cab7f838d9f9071d2fb8ccbf48bfL282) the logic for checking the `isReactComponent` prototype property has become more strict. It turns out we are using a few dependencies that are defining a method called `isReactComponent` and this update caused these components to be treated as an `IndeterminateComponent` type internally.

**If the current behavior is a bug, please provide the steps to reproduce and if possible a minimal demo of the problem. Your bug will get fixed much faster if we can run your code and it doesn't have dependencies other than React. Paste the link to your JSFiddle (https://jsfiddle.net/Luktwrdm/) or CodeSandbox (https://codesandbox.io/s/new) example below:**
https://codesandbox.io/s/v3qw39wky

**What is the expected behavior?**
It would be nice if React could recognize this case and maybe log an error in development. I'm happy to try implementing this if you want to go that route. People definitely shouldn't be defining `isReactComponent` on their own components, but if it happens a quick check and an error message would make it a quick fix.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**
Impacts only v16.5.0. It wasn't an issue due to the less exhaustive check in earlier versions.

/**
 * 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.
 *
 * @emails react-core
 */
'use strict';
let ChildUpdates;
let MorphingComponent;
let React;
let ReactDOM;
let ReactDOMServer;
let ReactCurrentOwner;
let ReactTestUtils;
let PropTypes;
describe('ReactCompositeComponent', () => {
  const hasOwnProperty = Object.prototype.hasOwnProperty;
  /**
   * Performs equality by iterating through keys on an object and returning false
   * when any key has values which are not strictly equal between the arguments.
   * Returns true when the values of all keys are strictly equal.
   */
  function shallowEqual(objA: mixed, objB: mixed): boolean {