In D207229#7114840, @gravyant wrote:

Am I on the right track with the function signature and implementation?

Yes, looks close :)

I know the method should be used only for classes with the isInstance method implemented, so should I check for the constructor having that method/property in the function body?

Yes, you have to add the check for the isInstance method.

Regarding the testing, it would be good in this case to add a xpcshell test here: https://searchfox.org/mozilla-central/source/remote/shared/webdriver/test/xpcshell/test_Assert.js