function MulticastObservable(source, fn1, fn2) {
      this.source = source;
      this._fn1 = fn1;
      this._fn2 = fn2;
      __super__.call(this);
    }
Invokes the asynchronous function, surfacing the result through an observable sequence.
@param {Function} functionAsync Asynchronous function which returns a Promise to run.
@returns {Observable} An observable sequence exposing the function's result value, or an exception.