Make verifiedLocale work differently depending on what it's verifying for

pkg/intl/lib/message_lookup_local.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Message/plural format library with locale support. This can have different
  * implementations based on the mechanism for finding the localized versions
  * of messages. This version expects them to be in a library named e.g.
  * 'messages_en_US'. The prefix is set in the [initializeMessages] call, which
  * must be made for a locale before any lookups can be done.
@@ skipping 40 matching lines @@
    *The [source] parameter defines the prefix that is used to find
    * libraries that contain localized messages. So with the default value of
    * 'messages_', we would look for messages for the locale 'pt_BR' in a library
    * named 'messages_pt_BR'.
    */
   MessageLookupLocal(String localeName, this._sourcePrefix) {
     _libraries = currentMirrorSystem().libraries;
   }
 
   /**
+   * Return true if the locale exists, or if it is null. The null case
+   * is interpreted to mean that we use the default locale.
+   */
+  bool localeExists(localeName) {
+    if (localeName == null) return false;
+    return _libraries['$_sourcePrefix$localeName'] != null;
+  }
+
+  /**
    * Return the localized version of a message. We are passed the original
    * version of the message, which consists of a
    * [message_str] that will be translated, and which may be interpolated
    * based on one or more variables, a [desc] providing a description of usage
    * for the [message_str], and a map of [examples] for each data element to be
    * substituted into the message.
    *
    * For example, if message="Hello, $name", then
    * examples = {'name': 'Sparky'}. If not using the user's default locale, or
    * if the locale is not easily detectable, explicitly pass [locale].
@@ skipping 11 matching lines @@
   String lookupMessage(String message_str, [final String desc='',
       final Map examples=const {}, String locale,
       String name, List<String> args]) {
     if (name == null) return message_str;
     // The translations also make use of Intl.message, so we need to not
     // recurse and just stop when we find the first substitution.
     if (_lookupInProgress) return message_str;
     _lookupInProgress = true;
     var result;
     try {
-      // TODO(alanknight): Look up alternate forms, e.g. en for en_US
       var actualLocale = (locale == null) ? Intl.getCurrentLocale() : locale;
+      // For this usage, if the locale doesn't exist for messages, just return
+      // it and we'll fall back to the original version.
+      var verifiedLocale =
+          Intl.verifiedLocale(
+              actualLocale,
+              localeExists,
+              onFailure: (locale)=>locale);
       LibraryMirror messagesForThisLocale =
-          _libraries['$_sourcePrefix$actualLocale'];
+          _libraries['$_sourcePrefix$verifiedLocale'];
       if (messagesForThisLocale == null) return message_str;
       MethodMirror localized = messagesForThisLocale.functions[name];
       if (localized == null) return message_str;
       result = messagesForThisLocale.invoke(localized.simpleName, args);
       }
     finally {
       _lookupInProgress = false;
     }
     return result.value.reflectee;
   }
 }