Dart APIunself_local_databaseFieldCompanionsystem system property Null safety Value<bool> system final Implementation final Value<bool> system; unself_local_database unself_local_database FieldCompanion system property FieldCompanion class Constructors FieldCompanion insert Properties collectionId created extra hashCode id name required runtimeType system type unique updated Methods copyWith noSuchMethod toColumns toString Operators operator == Static methods custom