code: function _findFileInMRUList(paneId, file) {
          return _.findIndex(_mruList, function (record) {
              return (record.file.fullPath === file.fullPath && record.paneId === paneId);
          });
      }
docs: 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