anchor.diff:    
     @override
     Future close() {
  -    return _conn.close().then((_) {
  -      _conn = null;
  -    });
  +    return new Future.value(null);
     }
   }
   
  
positive.code: 
  class Database {
    String _connStr;
    
    Database(String connectionString) 
        : _connStr = connectionString;
    
    Future<Connection> connect() {
      var db = new mdb.Db(_connStr);
      return db.open().then((_) => new _Connection(db));
    }
  }
  
  
negative.code: 
      cursor.skip = skip;
      cursor.limit = limit;
      
      var sc = new StreamController();
      
      return cursor.stream.asyncMap((m) {
        if(!_known.containsKey(m["_id"])) {
          var obj = _dty.mirror.newInstance(#revive, [m]).reflectee;
  