code: function(entries) {
          for ( var assetKey in entries ) {
              if ( entries.hasOwnProperty(assetKey) === false ) { continue; }
              entry = entries[assetKey];
              if ( entry.content !== 'filters' ) { continue; }
              newAvailableLists[assetKey] = Object.assign({}, entry);
          }
  
          // Load set of currently selected filter lists.
          var listKeySet = new Set(µb.selectedFilterLists);
          for ( listKey in newAvailableLists ) {
              if ( newAvailableLists.hasOwnProperty(listKey) ) {
                  newAvailableLists[listKey].off = !listKeySet.has(listKey);
              }
          }
  
          finalize();
          callback(newAvailableLists);
      }
docs: Built-in filter lists loaded.