Dart APIunself_modelFlatfieldMap fieldMap property Null safety Map<String, String> fieldMap Implementation Map<String, String> get fieldMap; unself_model unself_model Flat fieldMap property Flat class Constructors Flat fromJson Properties addMissingKeys copyWith definition entityPath extra fieldMap hashCode includePath key name runtimeType Methods map mapOrNull maybeMap maybeWhen noSuchMethod toJson toString when whenOrNull Operators operator ==