BeforeUnloadEvent tests return false negative in Edge 15-18

[foolip]

I have tested https://mdn-bcd-collector.gooborg.com/tests/api/BeforeUnloadEvent in Edge 15 and 18 in BrowserStack. The issue most likely affects Edge 12-18.

The test returns false with "Cannot create event". However, BeforeUnloadEvent is defined and 'returnValue' in BeforeUnloadEvent.prototype is true, so the event interface is supported.

Unlike other custom event tests, trying new BeforeUnloadEvent('beforeunload') won't work, because Edge throws with "Function expected".

I can't see a good way to write a test that will give the right result on all browsers, so filing this issue.