anchor.diff:    String collectionName;
     DbCollection(this.db, this.collectionName){}
     String fullName() => "${db.databaseName}.$collectionName";
  -  void save(Map document, {WriteConcern writeConcern}){
  +  Future save(Map document, {WriteConcern writeConcern}){
       var id;
       bool createId = false;
       if (document.containsKey("_id")){
  
positive.code:     return completer.future;
    }
    
    Future distinct(String field, [selector]){
      Completer completer = new Completer();
      db.executeDbCommand(DbCommand.createDistinctCommand(db,collectionName,field,_selectorBuilder2Map(selector))).then((reply){
        completer.complete(reply);
      });
      return completer.future;
    }
    
    Future aggregate(List pipeline){
      Completer completer = new Completer();
      var cmd = DbCommand.createAggregateCommand(db,collectionName,pipeline);
      db.executeDbCommand(cmd)
        .then((reply){
        completer.complete(reply);
      });
      
      return completer.future;
    }
    
    Future insert(Map document, {WriteConcern writeConcern}) => insertAll([document], writeConcern: writeConcern);
  
negative.code:     }
      return selector;
    }
  }
  