instanceof Event does not work as expected

[theseanl]

Steps to reproduce

1. Add a following userscript to AG for Win:

// ==UserScript==
// @name         instanceof
// @namespace    instanceof
// @match        *://*/*
// @grant        unsafeWindow
// ==/UserScript==

var a = new MouseEvent('click');
console.log(a instanceof Event);

2. Visit any site, open the console to see what is printed.

Expected behavior

The console prints true, as in Tampermonkey.

Actual behavior

The console prints false.

Your environment

Chrome 61
Windows 10 14393

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pull-requests/2