getRowIDForFlatIndex(index: number): ?string {
    var accessIndex = index;
    for (var ii = 0; ii < this.sectionIdentities.length; ii++) {
      if (accessIndex >= this.rowIdentities[ii].length) {
        accessIndex -= this.rowIdentities[ii].length;
      } else {
        return this.rowIdentities[ii][accessIndex];
      }
    }
    return null;
  }
Gets the rowID at index provided if the dataSource arrays were flattened,
or null of out of range indexes.