function FileIndex(indexName, filterFunction) {
        this.name = indexName;
        this.fileInfos = [];
        this.filterFunction = filterFunction;
    }
class FileIndex

A FileIndex contains an array of fileInfos that meet the criteria specified by
the filterFunction. FileInfo's in the fileInfo array should unique map to one file.
 
@constructor
@param {!string} indexname
@param {function({!entry})} filterFunction returns true to indicate the entry
                            should be included in the index