Re: [heycam/webidl] Remove [LegacyArrayClass] (#291)

> `a[0]` will work just fine for things with indexed getters, right?

It "works fine" by requiring a proxy to intercept all numeric property accesses, because TC39 hasn't gotten its head out of its ass and added a hook for [] syntax yet. :(  As such, it's not a solution we want to use in general.

> But yes, the lack of map() is just kinda silly. On the other hand, the lack of reverse() is not necessarily silly...

My use-case is dead lists, so there's nothing wrong with .reverse() or any of the other this-mutating functions.  But yeah, if we wanted to do this "properly", you'd want to separate out the list of mutating functions.

> filter (but what sort of object should this return?), map (again, what should this return?), 

Easy to define that the auto-defined versions of these methods return values of the same type, but I guess the problem is live collections, where it's not clear what the effect of such a thing would be?

I bet we could hinge it on whether there exists a constructor that takes a single `sequence<>` argument: if so, those methods automatically cast the result back into the type; if not, you get an Array back.  If you want an Array all the time, you can cast it yourself.

Or make it more explicit, with a variant `iterator` declaration that says whether the collection is live or dead.  (Maybe `arraylike<>` for the dead version?) And then require that `arraylike<>`s have an appropriate constructor to be valid.

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