IsConstructor

[André Bargull]

> I'd be most interested in seeing if we can remove IsConstructor entirely (except for uses where it's just a guard, implementing the semantics of `new` via IsConstructor -> [[Construct]] or throw).
>
> It seems like there's at least some movement toward removing it from `Array.of` and `Array.from`. All that remains is its use to preserve the `arrayInstance.constructor = undefined` backward-compatibility possibilities. My preference would be to see if we can get away with breaking that use case, and reintroduce it if that turns out not to be web-compatible.

The [[Realm]] check in Array.prototype.* is even more annoying than the 
IsConstructor guard, but unfortunately required for web-compatibility 
per [1]. :-(


[1] http://esdiscuss.org/topic/array-prototype-slice-web-compat-issue