Throw error when plural case had undefined behavior (#116622)

* init

* add comment

* make error more actionable

Author: thkim1011

packages/flutter_tools/lib/src/localizations/gen_l10n.dart

@@ -1167,7 +1167,20 @@ class LocalizationsGenerator {
             }
             if (!pluralLogicArgs.containsKey(pluralCases[pluralCase])) {
               final String pluralPartExpression = generateVariables(pluralMessage);
-              pluralLogicArgs[pluralCases[pluralCase]!] = '      ${pluralCases[pluralCase]}: $pluralPartExpression,';
+              final String? transformedPluralCase = pluralCases[pluralCase];
+              // A valid plural case is one of "=0", "=1", "=2", "zero", "one", "two", "few", "many", or "other".
+              if (transformedPluralCase == null) {
+                throw L10nParserException(
+                  '''
+The plural cases must be one of "=0", "=1", "=2", "zero", "one", "two", "few", "many", or "other.
+    $pluralCase is not a valid plural case.''',
+                  _inputFileNames[locale]!,
+                  message.resourceId,
+                  translationForMessage,
+                  pluralPart.positionInMessage,
+                );
+              }
+              pluralLogicArgs[transformedPluralCase] = '      ${pluralCases[pluralCase]}: $pluralPartExpression,';
             } else if (!suppressWarnings) {
               logger.printWarning('''
 [${_inputFileNames[locale]}:${message.resourceId}] ICU Syntax Warning: The plural part specified below is overridden by a later plural part.

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

@@ -2027,6 +2027,38 @@ import 'output-localization-file_en.dart' deferred as output-localization-file_e
                    ^'''));
       });
 
+      testWithoutContext('undefined plural cases throws syntax error', () {
+         const String pluralMessageWithUndefinedParts = '''
+{
+  "count": "{count,plural, =0{None} =1{One} =2{Two} =3{Undefined Behavior!} other{Hmm...}}"
+}''';
+        final Directory l10nDirectory = fs.currentDirectory.childDirectory('lib').childDirectory('l10n')
+          ..createSync(recursive: true);
+        l10nDirectory.childFile(defaultTemplateArbFileName)
+          .writeAsStringSync(pluralMessageWithUndefinedParts);
+        try {
+        LocalizationsGenerator(
+          fileSystem: fs,
+          inputPathString: defaultL10nPathString,
+          outputPathString: defaultL10nPathString,
+          templateArbFileName: defaultTemplateArbFileName,
+          outputFileString: defaultOutputFileString,
+          classNameString: defaultClassNameString,
+          logger: logger,
+        )
+          ..loadResources()
+          ..writeOutputFiles();
+        } on L10nException catch (error) {
+          expect(error.message, contains('Found syntax errors.'));
+          expect(logger.hadErrorOutput, isTrue);
+          expect(logger.errorText, contains('''
+[app_en.arb:count] The plural cases must be one of "=0", "=1", "=2", "zero", "one", "two", "few", "many", or "other.
+    3 is not a valid plural case.
+    {count,plural, =0{None} =1{One} =2{Two} =3{Undefined Behavior!} other{Hmm...}}
+                                            ^'''));
+        }
+      });
+
       testWithoutContext('should automatically infer plural placeholders that are not explicitly defined', () {
         const String pluralMessageWithoutPlaceholdersAttribute = '''
 {