Update stocks example to use l10n.yaml workflow (#58121)

dev/benchmarks/test_apps/stocks/l10n.yaml

+# Options used by the localizations tool
+## `arb-dir` sets the input directory. The output directory will match
+## the input directory if the output directory is not set.
+arb-dir: lib/i18n
+## `template-arb-file` describes the template arb file that the tool
+## will use to check and validate the remaining arb files when
+## generating Flutter's localization files.
+template-arb-file: stocks_en.arb
+## `output-localization-file` is the name of the generated file.
+output-localization-file: stock_strings.dart
+## `output-class` is the name of the localizations class your
+## Flutter application will use. The file will need to be
+## imported throughout your application.
+output-class: StockStrings
+## `header-file` is the file that contains a custom
+## header for each of the generated files.
+header-file: header.txt

dev/benchmarks/test_apps/stocks/lib/i18n/regenerate.md

 # Regenerating the i18n files
 
-The files in this directory are used to generate `stock_strings.dart`, which
-is used by the stocks application to look up localized message strings. The
-stocks app uses the [Dart `intl` package](https://github.com/dart-lang/intl).
+The files in this directory are used to generate `stock_strings.dart`,
+which contains the `StockStrings` class. This localizations class is
+used by the stocks application to look up localized message strings.
+The stocks app uses the [Dart `intl` package](https://github.com/dart-lang/intl).
 
-Rebuilding everything requires two steps.
-
-1. Create or update the English and Spanish localizations,
-`stocks_en_US.arb`, `stocks_en.arb`, and `stocks_es.arb`. See the
+To update the English and Spanish localizations, modify the
+`stocks_en_US.arb`, `stocks_en.arb`, or `stocks_es.arb` files. See the
 [ARB specification](https://github.com/google/app-resource-bundle/wiki/ApplicationResourceBundleSpecification)
 for more info.
 
-2. With `examples/stocks` as the current directory, generate a
-`messages_<locale>.dart` for each `stocks_<locale>.arb` file,
-`messages_all.dart`, and `stock_strings.dart` with the following command:
-
-```dart
-dart ${FLUTTER_PATH}/dev/tools/localization/bin/gen_l10n.dart --arb-dir=lib/i18n \
-    --template-arb-file=stocks_en.arb --output-localization-file=stock_strings.dart \
-    --output-class=StockStrings --header-file=header.txt
-```
+To modify the project's configuration of the localizations tool,
+change the `l10n.yaml` file.
 
 The `StockStrings` class creates a delegate that performs message lookups
 based on the locale of the device. In this case, the stocks app supports
-`en`, `en_US`, and `es`. Thus, the `StockStringsEn` and `StockStringsEs`
-classes extends `StockStrings`. `StockStringsEnUs` extends
+`en`, `en_US`, and `es` locales. Thus, the `StockStringsEn` and
+`StockStringsEs` classes extends `StockStrings`. `StockStringsEnUs` extends
 `StockStringsEn`. This allows `StockStringsEnUs` to fall back on messages
 in `StockStringsEn`.

dev/benchmarks/test_apps/stocks/lib/i18n/stock_strings.dart

@@ -4,17 +4,15 @@
 
 import 'dart:async';
 
+// ignore: unused_import
 import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 import 'package:flutter_localizations/flutter_localizations.dart';
-// ignore: unused_import
 import 'package:intl/intl.dart' as intl;
 
 import 'stock_strings_en.dart';
 import 'stock_strings_es.dart';
 
-// ignore_for_file: unnecessary_brace_in_string_interps
-
 /// Callers can lookup localized strings with an instance of StockStrings returned
 /// by `StockStrings.of(context)`.
 ///
@@ -42,8 +40,7 @@ import 'stock_strings_es.dart';
 ///   # Internationalization support.
 ///   flutter_localizations:
 ///     sdk: flutter
-///   intl: 0.16.0
-///   intl_translation: 0.17.7
+///   intl: 0.16.1
 ///
 ///   # rest of dependencies
 /// ```
@@ -99,7 +96,7 @@ abstract class StockStrings {
   /// A list of this localizations delegate's supported locales.
   static const List<Locale> supportedLocales = <Locale>[
     Locale('en'),
-    Locale('en, US'),
+    Locale('en', 'US'),
     Locale('es')
   ];
 
@@ -129,15 +126,24 @@ class _StockStringsDelegate extends LocalizationsDelegate<StockStrings> {
 }
 
 StockStrings _lookupStockStrings(Locale locale) {
-  switch(locale.languageCode) {
+
+
+  // Lookup logic when language+country codes are specified.
+  switch (locale.languageCode) {
     case 'en': {
       switch (locale.countryCode) {
         case 'US': return StockStringsEnUs();
       }
-      return StockStringsEn();
+      break;
+    }
   }
+
+  // Lookup logic when only language code is specified.
+  switch (locale.languageCode) {
+    case 'en': return StockStringsEn();
     case 'es': return StockStringsEs();
   }
+
   assert(false, 'StockStrings.delegate failed to load unsupported locale "$locale"');
   return null;
 }