**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.
typeof document !== 'undefined',
        'The `document` global was defined when React was initialized, but is not ' +
          'defined anymore. This can happen in a test environment if a component ' +
          'schedules an update from an asynchronous callback, but the test has already ' +
          'finished running. To solve this, you can either unmount the component at ' +
          'the end of your test (and ensure that any asynchronous operations get ' +
          'canceled in `componentWillUnmount`), or you can change the test itself ' +
          'to be asynchronous.',
      );
      const evt = document.createEvent('Event');
      
      
      
      
      
      
      let didError = true;
      
      
      
      let windowEvent = window.event;
      
      
      
      const funcArgs = Array.prototype.slice.call(arguments, 3);
      function callCallback() {