freezed

Put a discriminator field in the JSON, switch on it in a factory on the sealed base, and switch on the subtype to write it back. Three working versions on Dart 3.13.7: hand-written pattern matching, json_serializable 6.15.2, and Freezed 4.0.3 with unionKey and fallbackUnion, plus the gotchas each one hides.

Pick Dart 3.12 records for ephemeral, locally-shaped data with no methods, and Freezed 3.x classes for named domain models that need copyWith, sealed unions, JSON serialization, or any behaviour.