Helper function for finding the view-sequence node at the given position.
function _getViewSequenceAtIndex(index, startViewSequence) {
        if (this._viewSequence.getAtIndex) {
            return this._viewSequence.getAtIndex(index, startViewSequence);
        }
        var viewSequence = startViewSequence || this._viewSequence;
        var i = viewSequence ? viewSequence.getIndex() : index;
        if (index > i) {
            while (viewSequence) {
                viewSequence = viewSequence.getNext();
                if (!viewSequence) {
                    return undefined;
                }
                i = viewSequence.getIndex();
                if (i === index) {
                    return viewSequence;
                }
                else if (index < i) {
                    return undefined;
                }
            }
        }
        else if (index < i) {
            while (viewSequence) {
                viewSequence = viewSequence.getPrevious();
                if (!viewSequence) {
                    return undefined;
                }
                i = viewSequence.getIndex();
                if (i === index) {
                    return viewSequence;
                }
                else if (index > i) {
                    return undefined;
                }
            }
        }
        return viewSequence;
    }