Adds 'fallback' as a universal fallback locale for messages'

lib/intl.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.
 
 /**
  * This library provides internationalization and localization. This includes
  * message formatting and replacement, date and number formatting and parsing,
  * and utilities for working with Bidirectional text.
  *
  * This is part of the [intl package]
@@ skipping 170 matching lines @@
 
   /**
    * Return true if the locale exists, or if it is null. The null case
    * is interpreted to mean that we use the default locale.
    */
   static bool _localeExists(localeName) => DateFormat.localeExists(localeName);
 
   /**
    * Given [newLocale] return a locale that we have data for that is similar
    * to it, if possible.
+   *
    * If [newLocale] is found directly, return it. If it can't be found, look up
    * based on just the language (e.g. 'en_CA' -> 'en'). Also accepts '-'
    * as a separator and changes it into '_' for lookup, and changes the
    * country to uppercase.
+   *
+   * There is a special case that if a locale named "fallback" is present
+   * and has been initialized, this will return that name. This can be useful
+   * for messages where you don't want to just use the text from the original
+   * source code, but wish to have a universal fallback translation.
+   *
    * Note that null is interpreted as meaning the default locale, so if
    * [newLocale] is null it will be returned.
    */
   static String verifiedLocale(String newLocale, Function localeExists,
                                {Function onFailure: _throwLocaleError}) {
     // TODO(alanknight): Previously we kept a single verified locale on the Intl
     // object, but with different verification for different uses, that's more
     // difficult. As a result, we call this more often. Consider keeping
     // verified locales for each purpose if it turns out to be a performance
     // issue.
     if (newLocale == null) {
       return verifiedLocale(getCurrentLocale(), localeExists,
           onFailure: onFailure);
     }
     if (localeExists(newLocale)) {
       return newLocale;
     }
     for (var each in
-        [canonicalizedLocale(newLocale), shortLocale(newLocale)]) {
+        [canonicalizedLocale(newLocale), shortLocale(newLocale), "fallback"]) {
       if (localeExists(each)) {
         return each;
       }
     }
     return onFailure(newLocale);
   }
 
   /**
    * The default action if a locale isn't found in verifiedLocale. Throw
    * an exception indicating the locale isn't correct.
@@ skipping 162 matching lines @@
    * unless for some reason this gets called inside a message that resets the
    * locale.
    */
   static String getCurrentLocale() {
     if (defaultLocale == null) defaultLocale = systemLocale;
     return defaultLocale;
   }
 
   toString() => "Intl($locale)";
 }