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

Report a bug.

**What is the current behavior?**

Global `window.event` is overwritten in React 16.5+ in development mode. Here're minimal repro steps:

- React **16.5.2 in dev mode**: https://jsfiddle.net/sergei_startsev/ecz103vL/2/

If you click the button, you see `DOMContentLoaded` event type.

**What is the expected behavior?**

The current behavior contradicts with specified behavior [window property event returns the Event which is **currently being handled** by the site's code. Outside the context of an event handler, the value is **always** `undefined`](https://developer.mozilla.org/en-US/docs/Web/API/Window/event). Moreover it works properly in production mode:

- React **16.5.2 in prod mode**: https://jsfiddle.net/sergei_startsev/st5L8e02/2/

It returns expected `click` event type.

**Which versions of React, and which browser / OS are affected by this issue? Did this work in previous versions of React?**

It works properly in React 16.4.2 and prod mode:

- React **16.4.2** in dev mode: https://jsfiddle.net/sergei_startsev/o26L08qw/

The issue is reproduced in Chrome 69. It works properly in FF 62 (`window.event` isn't support in 62, however it should be reproduced in FF 63 - it was recently added, see [details](https://developer.mozilla.org/en-US/docs/Web/API/Window/event#Browser_Compatibility)).

It seems that the issue was introduced by @ConradIrwin in #11696.
if (
    typeof window !== 'undefined' &&
    typeof window.dispatchEvent === 'function' &&
    typeof document !== 'undefined' &&
    typeof document.createEvent === 'function'
  ) {
    const fakeNode = document.createElement('react');
    const invokeGuardedCallbackDev = function<A, B, C, D, E, F, Context>(
      name: string | null,
      func: (a: A, b: B, c: C, d: D, e: E, f: F) => mixed,
      context: Context,
      a: A,
      b: B,
      c: C,
      d: D,
      e: E,
      f: F,
    ) {
      
      
      
      
      invariant(