anchor.diff:  import 'package:pub_dartlang_org/frontend/service_utils.dart';
   
   final _argParser = new ArgParser()
  +  ..addOption('concurrency',
  +      abbr: 'c', defaultsTo: '1', help: 'Number of concurrent processing.')
     ..addOption('package', abbr: 'p', help: 'The package to backfill.')
     ..addFlag('help', abbr: 'h', defaultsTo: false, help: 'Show help.');
   
  +final _emailInProgress = <String, Completer>{};
   final _emailToUser = <String, User>{};
   final _userFirstUsed = <String, DateTime>{};
   
  
positive.code: 
  import 'package:args/args.dart';
  import 'package:gcloud/db.dart';
  
  import 'package:pub_dartlang_org/account/backend.dart';
  import 'package:pub_dartlang_org/account/models.dart';
  
negative.code: }
  
  Future<User> _lookupUserByEmail(String email) async {
    if (_emailInProgress.containsKey(email)) {
      await _emailInProgress[email].future;
    }
    if (_emailToUser.containsKey(email)) {
      return _emailToUser[email];
    }
    _emailInProgress[email] = Completer();
    final user = await accountBackend.lookupOrCreateUserByEmail(email);
    _emailToUser[email] = user;
    _emailInProgress.remove(email).complete();
    return user;
  }
  
  