function _findFileInMRUList(paneId, file) {
        return _.findIndex(_mruList, function (record) {
            return (record.file.fullPath === file.fullPath && record.paneId === paneId);
        });
    }
Locates the first  MRU entry of a file for the requested pane
@param {!string} paneId - the paneId
@param {!File} File - the file
@return {{file:File, paneId:string}}
@private