LANGUAGE_RULES had no entry for Dart, and FRAMEWORK_RULES had no way to
detect Flutter, so any Flutter app (pubspec.yaml, lib/*.dart, android/,
ios/) was reported as primary: "unknown" by the SessionStart project-type
banner, despite ECC shipping a dart-flutter-patterns skill and a
dart-build-resolver agent for this exact stack.
Adds a 'dart' language rule (pubspec.yaml/pubspec.lock markers, .dart
extension) and a 'flutter' framework rule that checks pubspec.yaml for
the standard `flutter: / sdk: flutter` dependency shape, via a new
customCheck hook on FRAMEWORK_RULES entries. This distinguishes Flutter
apps from plain Dart packages, which don't depend on the Flutter SDK.
Closes#3469
Beksultan Pirmatov
11a970e11e