make app localizations lookup a public method (#84732)

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

@@ -541,11 +541,11 @@ String _generateDelegateClass({
     useDeferredLoading ? loadBodyDeferredLoadingTemplate : loadBodyTemplate
   )
     .replaceAll('@(class)', className)
-    .replaceAll('@(lookupName)', '_lookup$className');
+    .replaceAll('@(lookupName)', 'lookup$className');
   final String lookupFunction = (useDeferredLoading ?
   lookupFunctionDeferredLoadingTemplate : lookupFunctionTemplate)
     .replaceAll('@(class)', className)
-    .replaceAll('@(lookupName)', '_lookup$className')
+    .replaceAll('@(lookupName)', 'lookup$className')
     .replaceAll('@(lookupBody)', lookupBody);
   return delegateClassTemplate
     .replaceAll('@(class)', className)

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

@@ -2449,4 +2449,30 @@ String orderNumber(int number) {
 '''));
     expect(localizationsFile, isNot(contains(intlImportDartCode)));
   });
+
+  testWithoutContext('app localizations lookup is a public method', () {
+    final Directory l10nDirectory = fs.currentDirectory.childDirectory('lib').childDirectory('l10n')
+      ..createSync(recursive: true);
+    l10nDirectory.childFile(defaultTemplateArbFileName)
+        .writeAsStringSync(singleMessageArbFileString);
+
+    LocalizationsGenerator(
+      fileSystem: fs,
+      inputPathString: defaultL10nPathString,
+      outputPathString: defaultL10nPathString,
+      templateArbFileName: defaultTemplateArbFileName,
+      outputFileString: defaultOutputFileString,
+      classNameString: defaultClassNameString,
+    )
+      ..loadResources()
+      ..writeOutputFiles(BufferLogger.test());
+
+    final String localizationsFile = fs.file(
+      fs.path.join(syntheticL10nPackagePath, 'output-localization-file.dart'),
+    ).readAsStringSync();
+    print(localizationsFile);
+    expect(localizationsFile, containsIgnoringWhitespace(r'''
+AppLocalizations lookupAppLocalizations(Locale locale) {
+'''));
+  });
 }