function AsyncLoop(iterations, _fn, _successCallback, offset) {
            if (offset === void 0) { offset = 0; }
            this.iterations = iterations;
            this._fn = _fn;
            this._successCallback = _successCallback;
            this.index = offset - 1;
            this._done = false;
        }
Constroctor.
@param iterations the number of iterations.
@param _fn the function to run each iteration
@param _successCallback the callback that will be called upon succesful execution
@param offset starting offset.