It should be impossible to construct an AbortSignal manually

[benjamingr]

In AbortController/AbortSignal it is possible to construct an AbortSignal without a controller (but shouldn't be):

> var c = new AbortController();
> new c.signal.constructor()
AbortSignal { aborted: false }

Correct behavior:

> var c = new AbortController();
new c.signal.constructor()
TypeError: Illegal Constructor

This should be a relatively simple fix but it's a spec compliance issue - so if no new contributor picks this up I'll fix this sometime next week :]

[dev-script]

@benjamingr , I would like to work on this issue

[benjamingr]

Thanks @RaisinTen !

[RaisinTen]

@benjamincburns you're welcome. 🙂