[flutter_tools] For l10n with deferred loading, use loadLibrary for non-web too (#59539)

After Dart VM change we are now required to use loadLibrary on an import whenever it is imported as deferred.

See: https://dart-review.googlesource.com/c/sdk/+/149613

dev/tools/localization/gen_l10n_templates.dart

@@ -217,19 +217,6 @@ const String lookupFunctionTemplate = '''
 }''';
 
 const String lookupFunctionDeferredLoadingTemplate = '''
-/// Lazy load the library for web, on other platforms we return the
-/// localizations synchronously.
-Future<@(class)> _loadLibraryForWeb(
-  Future<dynamic> Function() loadLibrary,
-  @(class) Function() localizationClosure,
-) {
-  if (kIsWeb) {
-    return loadLibrary().then((dynamic _) => localizationClosure());
-  } else {
-    return SynchronousFuture<@(class)>(localizationClosure());
-  }
-}
-
 Future<@(class)> @(lookupName)(Locale locale) {
   @(lookupBody)
   assert(false, '@(class).delegate failed to load unsupported locale "\$locale"');
@@ -243,7 +230,7 @@ const String lookupBodyTemplate = '''@(lookupAllCodesSpecified)
 
 const String switchClauseTemplate = '''case '@(case)': return @(localeClass)();''';
 
-const String switchClauseDeferredLoadingTemplate = '''case '@(case)': return _loadLibraryForWeb(@(library).loadLibrary, () => @(library).@(localeClass)());''';
+const String switchClauseDeferredLoadingTemplate = '''case '@(case)': return @(library).loadLibrary().then((dynamic _) => @(library).@(localeClass)());''';
 
 const String nestedSwitchTemplate = '''case '@(languageCode)': {
       switch (locale.@(code)) {

dev/tools/test/localization/gen_l10n_test.dart

@@ -1006,7 +1006,7 @@ import 'output-localization-file_zh.dart';
 '''));
     });
 
-    test('imports are deferred when useDeferredImports are set', () {
+    test('imports are deferred and loaded when useDeferredImports are set', () {
       fs.currentDirectory.childDirectory('lib').childDirectory('l10n')..createSync(recursive: true)
         ..childFile(defaultTemplateArbFileName).writeAsStringSync(singleMessageArbFileString);
 
@@ -1033,6 +1033,7 @@ import 'output-localization-file_zh.dart';
 '''
 import 'output-localization-file_en.dart' deferred as output-localization-file_en;
 '''));
+      expect(localizationsFile, contains('output-localization-file_en.loadLibrary()'));
     });
 
     group('DateTime tests', () {

packages/flutter_tools/test/integration.shard/test_data/gen_l10n_project.dart

@@ -94,7 +94,7 @@ class Home extends StatelessWidget {
     final List<String> results = [];
     return Row(
       children: <Widget>[
-        ResultBuilder(
+        LocaleBuilder(
           test: 'supportedLocales',
           callback: (BuildContext context) {
             results.add('--- supportedLocales tests ---');
@@ -254,8 +254,8 @@ class Home extends StatelessWidget {
             ]);
           },
         ),
-        Builder(
-          builder: (BuildContext context) {
+        LocaleBuilder(
+          callback: (BuildContext context) {
             try {
               int n = 0;
               for (final String result in results) {