[gen_l10n] Fix unintended breaking change introduced by output-dir option (#55909)

* Fix breaking change introduced by gen_l10n output-dir option

dev/tools/localization/bin/gen_l10n.dart

@@ -27,10 +27,10 @@ void main(List<String> arguments) {
   );
   parser.addOption(
     'output-dir',
-    defaultsTo: path.join('lib', 'l10n'),
     help: 'The directory where the generated localization classes will be written. '
       'The app must import the file specified in the \'output-localization-file\' '
-      'option from this directory.'
+      'option from this directory. If unspecified, this defaults to the same '
+      'directory as the input directory specified in \'arb-dir\'.'
   );
   parser.addOption(
     'template-arb-file',

dev/tools/localization/gen_l10n.dart

@@ -511,7 +511,7 @@ class LocalizationsGenerator {
     bool useDeferredLoading = false,
   }) {
     setInputDirectory(inputPathString);
-    setOutputDirectory(outputPathString);
+    setOutputDirectory(outputPathString ?? inputPathString);
     setTemplateArbFile(templateArbFileName);
     setOutputFile(outputFileString);
     setPreferredSupportedLocales(preferredSupportedLocaleString);

dev/tools/test/localization/gen_l10n_test.dart

@@ -346,6 +346,30 @@ void main() {
     expect(unimplementedOutputString, contains('subtitle'));
   });
 
+  test('uses inputPathString as outputPathString when the outputPathString is null', () {
+    _standardFlutterDirectoryL10nSetup(fs);
+    LocalizationsGenerator generator;
+    try {
+      generator = LocalizationsGenerator(fs);
+      generator
+        ..initialize(
+          inputPathString: defaultL10nPathString,
+          templateArbFileName: defaultTemplateArbFileName,
+          outputFileString: defaultOutputFileString,
+          classNameString: defaultClassNameString,
+        )
+        ..loadResources()
+        ..writeOutputFile();
+    } on L10nException catch (e) {
+      fail('Generating output should not fail: \n${e.message}');
+    }
+
+    final Directory outputDirectory = fs.directory('lib').childDirectory('l10n');
+    expect(outputDirectory.childFile('output-localization-file.dart').existsSync(), isTrue);
+    expect(outputDirectory.childFile('output-localization-file_en.dart').existsSync(), isTrue);
+    expect(outputDirectory.childFile('output-localization-file_es.dart').existsSync(), isTrue);
+  });
+
   test('correctly generates output files in non-default output directory if it already exists', () {
   final Directory l10nDirectory = fs.currentDirectory
     .childDirectory('lib')