Test and fix https://bugs.dojotoolkit.org/ticket/18637

[kfranqueiro]

Given that IINM a lot of the aspect code in Dojo 2 was brought straight over from Dojo 1, I suspect it has the same issue here. We should port over this fix if the problem exists.

I'd also ideally like to make the aspect code a lot easier to follow in Dojo 2, but that will likely be another future ticket.

[kfranqueiro]

I've confirmed locally with new tests that the same problem exists in dojo-core. In the process I also discovered that our typings for before did not permit for advising functions to not return anything, and we were clearly not testing that case either (since compilation would've failed if we were). I'll be addressing both of these issues at once.

WIP on my aspect-remove branch.

[kfranqueiro]

dojo-core's aspect actually had a different issue/failure with the same tests as the Dojo 1 issue. It was not nullifying advised.advice, which left no indication of "dead" advice handles, and actually caused removed advice functions to still be called in some cases.

This has been fixed in 22db350. (I forgot to update my commit message to reference this issue before merging, whoops.)