anchor.diff:  
   import '../../package/backend.dart';
   import '../../package/models.dart';
  +import '../../shared/datastore_helper.dart';
   
   final _logger = Logger('backfill.packageversions');
   
  
positive.code:   final existingAssetKeys =
        await existingAssetQuery.run().map((a) => a.key).toList();
  
    return await dbService.withTransaction((tx) async {
      final pvPubspec = await tx.lookupValue<PackageVersionPubspec>(
          dbService.emptyKey.append(PackageVersionPubspec,
              id: derived.packageVersionPubspec.id),
  
negative.code:     if (inserts.isNotEmpty || deletes.isNotEmpty) {
        tx.queueMutations(inserts: inserts, deletes: deletes);
      }
      return BackfillStat(
        versionCount: 1,
        pvPubspecCount: inserts.whereType<PackageVersionPubspec>().length,
  