function IndexOfObservable(source, e, n) {
      this.source = source;
      this._e = e;
      this._n = n;
      __super__.call(this);
    }
Returns an observable sequence containing a value that represents how many elements in the specified observable sequence satisfy a condition if provided, else the count of items.
@example
res = source.count();
res = source.count(function (x) { return x > 3; });
@param {Function} [predicate]A function to test each element for a condition.
@param {Any} [thisArg] Object to use as this when executing callback.
@returns {Observable} An observable sequence containing a single element with a number that represents how many elements in the input sequence satisfy the condition in the predicate function if provided, else the count of items in the sequence.