Add type validation to non-template .arb file parsing logic (#139035)

Resolves https://github.com/flutter/flutter/issues/138297. 

When reading from a .arb file (which contains localizations of strings in the form of a user-defined JSON string-string map), validate the type of values that we read.

See [this comment of mine on #138297](https://github.com/flutter/flutter/issues/138297#issuecomment-1827043260) to see how I arrived at this fix.

packages/flutter_tools/lib/src/localizations/gen_l10n_types.dart

@@ -4,6 +4,7 @@
 
 import 'package:intl/locale.dart';
 
+import '../base/common.dart';
 import '../base/file_system.dart';
 import '../base/logger.dart';
 import '../convert.dart';
@@ -646,7 +647,14 @@ class AppResourceBundle {
   final Map<String, Object?> resources;
   final Iterable<String> resourceIds;
 
-  String? translationFor(String resourceId) => resources[resourceId] as String?;
+  String? translationFor(String resourceId) {
+    final Object? result = resources[resourceId];
+    if (result is! String?) {
+      throwToolExit('Localized message for key "$resourceId" in "${file.path}" '
+        'is not a string.');
+    }
+    return result;
+  }
 
   @override
   String toString() {

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

@@ -1241,6 +1241,34 @@ class AppLocalizationsEn extends AppLocalizations {
         )),
       );
     });
+
+    testWithoutContext('AppResourceBundle throws if file contains non-string value', () {
+      const String inputPathString = 'lib/l10n';
+      const String templateArbFileName = 'app_en.arb';
+      const String outputFileString = 'app_localizations.dart';
+      const String classNameString = 'AppLocalizations';
+
+      fs.file(fs.path.join(inputPathString, templateArbFileName))
+        ..createSync(recursive: true)
+        ..writeAsStringSync('{ "helloWorld": "Hello World!" }');
+      fs.file(fs.path.join(inputPathString, 'app_es.arb'))
+        ..createSync(recursive: true)
+        ..writeAsStringSync('{ "helloWorld": {} }');
+
+      final LocalizationsGenerator generator = LocalizationsGenerator(
+        fileSystem: fs,
+        inputPathString: inputPathString,
+        templateArbFileName: templateArbFileName,
+        outputFileString: outputFileString,
+        classNameString: classNameString,
+        logger: logger,
+      );
+      expect(
+          () => generator.loadResources(),
+          throwsToolExit(message: 'Localized message for key "helloWorld" in '
+            '"lib/l10n/app_es.arb" is not a string.'),
+      );
+    });
   });
 
   group('writeOutputFiles', () {