function _addIndex(indexName, filterFunction) {
        if (_indexList.hasOwnProperty(indexName)) {
            throw new Error("Duplicate index name");
        }
        if (typeof filterFunction !== "function") {
            throw new Error("Invalid arguments");
        }

        _indexList[indexName] = new FileIndex(indexName, filterFunction);

        _indexListDirty = true;
    }
Adds a new index to _indexList and marks the list dirty 

A future performance optimization is to only build the new index rather than 
marking them all dirty

@private
@param {!string} indexName must be unque
@param {!function({entry} filterFunction should return true to include an
  entry in the index