/*
See https://github.com/ReactiveX/rxjs/blob/5.0.2/src/util/subscribeToResult.ts#L70-L77
And also: 
- https://github.com/ReactiveX/rxjs/blob/5.0.2/src/Observable.ts
- https://github.com/ReactiveX/rxjs/blob/5.0.2/src/symbol/observable.ts
*/
if (result instanceof Observable) {
  if (result._isScalar) {
    destination.next((<any>result).value);
    destination.complete();
    return null;
  } else {
    return result.subscribe(destination);
  }
}
// ...
if (result && typeof result[$$observable] === 'function') {
  const obs = result[$$observable]();
  if (typeof obs.subscribe !== 'function') {
    destination.error(new TypeError('Provided object does not correctly implement Symbol.observable'));
  } else {
    return obs.subscribe(new InnerSubscriber(outerSubscriber, outerValue, outerIndex));
  }
}