Add Symbol.observable to Observable interface

[appsforartists]

Can't add the [$$observable] line of

export interface Observable<T> {
  subscribe(listener: Observer | Next): Subscription;
  [$$observable](): Observable<T>;
}

because $$observable throws this error:

TS1169: A computed property name in an interface must directly refer to a built-in symbol.

Symbol didn't exist until Safari 8, so using Symbol.observable rather than $$observable would affect our compatibility with Safari 8 for no user-justifiable reason.

[appsforartists]

microsoft/TypeScript#5579 might be related.

[appsforartists]

Using Symbol also make the polyfill eligible for dead code elimination, so you end up writing something like

if (typeof Symbol !== 'function') {
  IO.prototype[$$observable] = function() { 
    return this; 
  }
}

which would never run, because Symbol.observable would be a reference error before you even got there.

[appsforartists]

This has been fixed for a while.