Dart APIunself_modelDocumentextra extra property Null safety @JsonExtra() Map<String, dynamic> extra inherited Implementation @JsonExtra() Map<String, dynamic> get extra => throw _privateConstructorUsedError; unself_model unself_model Document extra property Document class Constructors Document fromJson Properties archive collectionId collectionName copyWith created extra hashCode id runtimeType updated Methods noSuchMethod toJson toString Operators operator ==