Agent Skills: dart-language

Dart 3.x language feature standards: null safety, records, sealed classes, switch pattern matching, extensions, and async/await. ALWAYS consult when using !, ?., ??, late, sealed classes, record types, switch expressions, or async patterns — and before introducing any new Dart 3.x construct to confirm the modern idiomatic approach. (triggers: **/*.dart, sealed, record, switch, pattern, !, late, async, extension)