[gen_l10n] retain full output file suffix (#88362)

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

@@ -1167,6 +1167,12 @@ class LocalizationsGenerator {
 
     final String directory = _fs.path.basename(outputDirectory.path);
     final String outputFileName = _fs.path.basename(baseOutputFile.path);
+    if (!outputFileName.endsWith('.dart')) {
+      throw L10nException(
+        "The 'output-localization-file', $outputFileName, is invalid.\n"
+        'The file name must have a .dart extension.'
+      );
+    }
 
     final Iterable<String> supportedLocalesCode = supportedLocales.map((LocaleInfo locale) {
       final String languageCode = locale.languageCode;
@@ -1189,10 +1195,18 @@ class LocalizationsGenerator {
     );
 
     final List<LocaleInfo> allLocales = _allBundles.locales.toList()..sort();
-    final String fileName = outputFileName.split('.')[0];
+    final int extensionIndex = outputFileName.indexOf('.');
+    if (extensionIndex <= 0) {
+      throw L10nException(
+        "The 'output-localization-file', $outputFileName, is invalid.\n"
+        'The base name cannot be empty.'
+      );
+    }
+    final String fileName = outputFileName.substring(0, extensionIndex);
+    final String fileExtension = outputFileName.substring(extensionIndex + 1);
     for (final LocaleInfo locale in allLocales) {
       if (isBaseClassLocale(locale, locale.languageCode)) {
-        final File languageMessageFile = outputDirectory.childFile('${fileName}_$locale.dart');
+        final File languageMessageFile = outputDirectory.childFile('${fileName}_$locale.$fileExtension');
 
         // Generate the template for the base class file. Further string
         // interpolation will be done to determine if there are
@@ -1229,9 +1243,9 @@ class LocalizationsGenerator {
       .map((LocaleInfo locale) {
         final String library = '${fileName}_${locale.toString()}';
         if (useDeferredLoading) {
-          return "import '$library.dart' deferred as $library;";
+          return "import '$library.$fileExtension' deferred as $library;";
         } else {
-          return "import '$library.dart';";
+          return "import '$library.$fileExtension';";
         }
       })
       .toList()

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

@@ -751,7 +751,7 @@ void main() {
         arbDirectory: Uri.directory(defaultL10nPathString),
         deferredLoading: true,
         outputClass: 'Foo',
-        outputLocalizationsFile: Uri.file('bar', windows: false),
+        outputLocalizationsFile: Uri.file('bar.dart', windows: false),
         outputDirectory: Uri.directory(defaultL10nPathString, windows: false),
         preferredSupportedLocales: <String>['es'],
         templateArbFile: Uri.file(defaultTemplateArbFileName, windows: false),
@@ -773,7 +773,7 @@ void main() {
       expect(generator.inputDirectory.path, '/lib/l10n/');
       expect(generator.outputDirectory.path, '/lib/l10n/');
       expect(generator.templateArbFile.path, '/lib/l10n/app_en.arb');
-      expect(generator.baseOutputFile.path, '/lib/l10n/bar');
+      expect(generator.baseOutputFile.path, '/lib/l10n/bar.dart');
       expect(generator.className, 'Foo');
       expect(generator.preferredSupportedLocales.single, LocaleInfo.fromString('es'));
       expect(generator.header, 'HEADER');
@@ -790,7 +790,7 @@ void main() {
 HEADER
 
 
-import 'bar';
+import 'bar.dart';
 
 /// The translations for English (`en`).
 class FooEn extends Foo {
@@ -1322,6 +1322,94 @@ import 'output-localization-file_zh.dart';
 '''));
     });
 
+    // Regression test for https://github.com/flutter/flutter/issues/88356
+    testWithoutContext('full output file suffix is retained', () {
+      fs.currentDirectory.childDirectory('lib').childDirectory('l10n')..createSync(recursive: true)
+        ..childFile(defaultTemplateArbFileName).writeAsStringSync(singleMessageArbFileString);
+
+      LocalizationsGenerator(
+        fileSystem: fs,
+        inputPathString: defaultL10nPathString,
+        outputPathString: defaultL10nPathString,
+        templateArbFileName: defaultTemplateArbFileName,
+        outputFileString: 'output-localization-file.g.dart',
+        classNameString: defaultClassNameString,
+      )
+        ..loadResources()
+        ..writeOutputFiles(BufferLogger.test());
+
+      final String baseLocalizationsFile = fs.file(
+        fs.path.join(syntheticL10nPackagePath, 'output-localization-file.g.dart'),
+      ).readAsStringSync();
+      expect(baseLocalizationsFile, contains(
+'''
+import 'output-localization-file_en.g.dart';
+'''));
+
+      final String englishLocalizationsFile = fs.file(
+        fs.path.join(syntheticL10nPackagePath, 'output-localization-file_en.g.dart'),
+      ).readAsStringSync();
+      expect(englishLocalizationsFile, contains(
+'''
+import 'output-localization-file.g.dart';
+'''));
+    });
+
+    testWithoutContext('throws an exception when invalid output file name is passed in', () {
+      fs.currentDirectory.childDirectory('lib').childDirectory('l10n')..createSync(recursive: true)
+        ..childFile(defaultTemplateArbFileName).writeAsStringSync(singleMessageArbFileString);
+
+        expect(
+          () {
+            LocalizationsGenerator(
+              fileSystem: fs,
+              inputPathString: defaultL10nPathString,
+              outputPathString: defaultL10nPathString,
+              templateArbFileName: defaultTemplateArbFileName,
+              outputFileString: 'asdf',
+              classNameString: defaultClassNameString,
+            )
+              ..loadResources()
+              ..writeOutputFiles(BufferLogger.test());
+          },
+          throwsA(isA<L10nException>().having(
+            (L10nException e) => e.message,
+            'message',
+            allOf(
+              contains('output-localization-file'),
+              contains('asdf'),
+              contains('is invalid'),
+              contains('The file name must have a .dart extension.'),
+            ),
+          )),
+        );
+
+        expect(
+          () {
+            LocalizationsGenerator(
+              fileSystem: fs,
+              inputPathString: defaultL10nPathString,
+              outputPathString: defaultL10nPathString,
+              templateArbFileName: defaultTemplateArbFileName,
+              outputFileString: '.g.dart',
+              classNameString: defaultClassNameString,
+            )
+              ..loadResources()
+              ..writeOutputFiles(BufferLogger.test());
+          },
+          throwsA(isA<L10nException>().having(
+            (L10nException e) => e.message,
+            'message',
+            allOf(
+              contains('output-localization-file'),
+              contains('.g.dart'),
+              contains('is invalid'),
+              contains('The base name cannot be empty.'),
+            ),
+          )),
+        );
+      });
+
     testWithoutContext('imports are deferred and loaded when useDeferredImports are set', () {
       fs.currentDirectory.childDirectory('lib').childDirectory('l10n')..createSync(recursive: true)
         ..childFile(defaultTemplateArbFileName).writeAsStringSync(singleMessageArbFileString);