function TakeWhileObservable(source, fn) {
      this.source = source;
      this._fn = fn;
      __super__.call(this);
    }
Returns a specified number of contiguous elements from the start of an observable sequence, using the specified scheduler for the edge case of take(0).
@param {Number} count The number of elements to return.
@param {Scheduler} [scheduler] Scheduler used to produce an OnCompleted message in case <paramref name="count count</paramref> is set to 0.
@returns {Observable} An observable sequence that contains the specified number of elements from the start of the input sequence.