[gen_l10n] Throw a specified exception when the language code is invalid (#102431)

packages/flutter_tools/lib/src/localizations/localizations_utils.dart

@@ -7,6 +7,7 @@ import 'package:yaml/yaml.dart';
 
 import '../base/file_system.dart';
 import '../base/logger.dart';
+import 'gen_l10n_types.dart';
 import 'language_subtag_registry.dart';
 
 typedef HeaderGenerator = String Function(String regenerateInstructions);
@@ -215,8 +216,15 @@ void precacheLanguageAndRegionTags() {
 String describeLocale(String tag) {
   final List<String> subtags = tag.split('_');
   assert(subtags.isNotEmpty);
-  assert(_languages.containsKey(subtags[0]));
-  final String language = _languages[subtags[0]]!;
+  final String languageCode = subtags[0];
+  if (!_languages.containsKey(languageCode)) {
+    throw L10nException(
+      '"$languageCode" is not a supported language code.\n'
+      'See https://www.iana.org/assignments/language-subtag-registry/language-subtag-registry '
+      'for the supported list.',
+    );
+  }
+  final String language = _languages[languageCode]!;
   String output = language;
   String? region;
   String? script;

packages/flutter_tools/test/general.shard/generate_localizations_test.dart

@@ -2595,6 +2595,39 @@ import 'output-localization-file_en.dart' deferred as output-localization-file_e
         );
       });
     });
+
+    testWithoutContext('throws when the language code is not supported', () {
+      const String arbFileWithInvalidCode = '''
+{
+  "@@locale": "invalid",
+  "title": "invalid"
+}''';
+
+      final Directory l10nDirectory = fs.currentDirectory.childDirectory('lib').childDirectory('l10n')
+        ..createSync(recursive: true);
+      l10nDirectory.childFile('app_invalid.arb')
+        .writeAsStringSync(arbFileWithInvalidCode);
+
+      expect(
+        () {
+          LocalizationsGenerator(
+            fileSystem: fs,
+            inputPathString: defaultL10nPathString,
+            outputPathString: defaultL10nPathString,
+            templateArbFileName: 'app_invalid.arb',
+            outputFileString: defaultOutputFileString,
+            classNameString: defaultClassNameString,
+          )
+            ..loadResources()
+            ..writeOutputFiles(BufferLogger.test());
+        },
+        throwsA(isA<L10nException>().having(
+          (L10nException e) => e.message,
+          'message',
+          contains('"invalid" is not a supported language code.'),
+        )),
+      );
+    });
   });
 
   testWithoutContext('should generate a valid pubspec.yaml file when using synthetic package if it does not already exist', () {