SERVER-38769, rollback would not attempt to fix collection counts for drop-pending collections managed by the storage engine. This is because the drop-pending collection has been removed from the catalog while the underlying ident is kept around for rollback/recovery. The collection counts are eventually fixed coming out of rollback because rollback rebuilds the indexes on drop-pending collections in catalog::openCatalog().
SERVER-38548 is completed, dropping indexes will defer the corresponding ident drops in a similar manner to what's currently done for collections. We should look into an alternate means of reconciling collection counts during rollback if we cannot rely on index rebuilds.