Observable: Subscription and SubscriptionObserver prototypes lack a closed getter.

[jasmith79]

Per the proposal both of the following console.logs should work (simplified example):

let obs = new Observable(observer => { console.log(observer.closed); }); // undefined
let subs = obs.subscribe({});
console.log(subs.closed);  // undefined

console.log("closed" in subs); // false

This is part of the test conformance suite and it should return false or true depending. Looking through the source code it doesn't seem implemented. Seems like a trivial PR if you'd like me to do it.

[zloirock]

I'll try to add it to core-js@3 because actual Observable has many breaking changes.

[zloirock]

Added to v3 branch.