[gen_l10n] Better blank lines in the header of generated files (#103414)

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

@@ -1133,14 +1133,14 @@ class LocalizationsGenerator {
     });
 
     return classFileTemplate
-      .replaceAll('@(header)', header)
+      .replaceAll('@(header)', header.isEmpty ? '' : '$header\n\n')
       .replaceAll('@(language)', describeLocale(locale.toString()))
       .replaceAll('@(baseClass)', className)
       .replaceAll('@(fileName)', fileName)
       .replaceAll('@(class)', '$className${locale.camelCase()}')
       .replaceAll('@(localeName)', locale.toString())
       .replaceAll('@(methods)', methods.join('\n\n'))
-      .replaceAll('@(requiresIntlImport)', _requiresIntlImport() ? "import 'package:intl/intl.dart' as intl;" : '');
+      .replaceAll('@(requiresIntlImport)', _requiresIntlImport() ? "import 'package:intl/intl.dart' as intl;\n\n" : '');
   }
 
   String _generateSubclass(
@@ -1284,7 +1284,7 @@ class LocalizationsGenerator {
     );
 
     return fileTemplate
-      .replaceAll('@(header)', header)
+      .replaceAll('@(header)', header.isEmpty ? '' : '$header\n')
       .replaceAll('@(class)', className)
       .replaceAll('@(methods)', _allMessages.map((Message message) => generateBaseClassMethod(message, _templateArbLocale)).join('\n'))
       .replaceAll('@(importFile)', '$directory/$outputFileName')

packages/flutter_tools/lib/src/localizations/gen_l10n_templates.dart

@@ -9,8 +9,7 @@ description: The Flutter application's synthetic package.
 ''';
 
 const String fileTemplate = '''
-@(header)
-import 'dart:async';
+@(header)import 'dart:async';
 
 @(requiresFoundationImport)
 import 'package:flutter/widgets.dart';
@@ -19,14 +18,14 @@ import 'package:intl/intl.dart' as intl;
 
 @(messageClassImports)
 
-/// Callers can lookup localized strings with an instance of @(class) returned
-/// by `@(class).of(context)`.
+/// Callers can lookup localized strings with an instance of @(class)
+/// returned by `@(class).of(context)`.
 ///
 /// Applications need to include `@(class).delegate()` in their app's
-/// localizationDelegates list, and the locales they support in the app's
-/// supportedLocales list. For example:
+/// `localizationDelegates` list, and the locales they support in the app's
+/// `supportedLocales` list. For example:
 ///
-/// ```
+/// ```dart
 /// import '@(importFile)';
 ///
 /// return MaterialApp(
@@ -41,14 +40,14 @@ import 'package:intl/intl.dart' as intl;
 /// Please make sure to update your pubspec.yaml to include the following
 /// packages:
 ///
-/// ```
+/// ```yaml
 /// dependencies:
 ///   # Internationalization support.
 ///   flutter_localizations:
 ///     sdk: flutter
 ///   intl: any # Use the pinned version from flutter_localizations
 ///
-///   # rest of dependencies
+///   # Rest of dependencies
 /// ```
 ///
 /// ## iOS Applications
@@ -202,10 +201,7 @@ const String selectMethodTemplateInString = '''
   }''';
 
 const String classFileTemplate = '''
-@(header)
-
-@(requiresIntlImport)
-import '@(fileName)';
+@(header)@(requiresIntlImport)import '@(fileName)';
 
 /// The translations for @(language) (`@(localeName)`).
 class @(class) extends @(baseClass) {

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

@@ -789,7 +789,6 @@ void main() {
       expect(fs.file('/lib/l10n/bar_en.dart').readAsStringSync(), '''
 HEADER
 
-
 import 'bar.dart';
 
 /// The translations for English (`en`).
@@ -839,6 +838,65 @@ flutter:
         ),
       );
     });
+
+    testWithoutContext('blank lines generated nicely', () async {
+      _standardFlutterDirectoryL10nSetup(fs);
+
+      // Test without headers.
+      generateLocalizations(
+        fileSystem: fs,
+        options: LocalizationOptions(
+          arbDirectory: Uri.directory(defaultL10nPathString),
+          outputDirectory: Uri.directory(defaultL10nPathString, windows: false),
+          templateArbFile: Uri.file(defaultTemplateArbFileName, windows: false),
+          useSyntheticPackage: false,
+        ),
+        logger: BufferLogger.test(),
+        projectDir: fs.currentDirectory,
+        dependenciesDir: fs.currentDirectory,
+      );
+
+      expect(fs.file('/lib/l10n/app_localizations_en.dart').readAsStringSync(), '''
+import 'app_localizations.dart';
+
+/// The translations for English (`en`).
+class AppLocalizationsEn extends AppLocalizations {
+  AppLocalizationsEn([String locale = 'en']) : super(locale);
+
+  @override
+  String get title => 'Title';
+}
+''');
+
+    // Test with headers.
+    generateLocalizations(
+      fileSystem: fs,
+      options: LocalizationOptions(
+        header: 'HEADER',
+        arbDirectory: Uri.directory(defaultL10nPathString),
+        outputDirectory: Uri.directory(defaultL10nPathString, windows: false),
+        templateArbFile: Uri.file(defaultTemplateArbFileName, windows: false),
+        useSyntheticPackage: false,
+      ),
+      logger: BufferLogger.test(),
+      projectDir: fs.currentDirectory,
+      dependenciesDir: fs.currentDirectory,
+    );
+
+    expect(fs.file('/lib/l10n/app_localizations_en.dart').readAsStringSync(), '''
+HEADER
+
+import 'app_localizations.dart';
+
+/// The translations for English (`en`).
+class AppLocalizationsEn extends AppLocalizations {
+  AppLocalizationsEn([String locale = 'en']) : super(locale);
+
+  @override
+  String get title => 'Title';
+}
+''');
+    });
   });
 
   group('loadResources', () {