Make withLocale use a zone

README.md

 Intl
 ====
 
 This package provides internationalization and localization facilities,
 including message translation, plurals and genders, date/number formatting
 and parsing, and bidirectional text.
 
 ## General
 The most important library is [intl][intl_lib]. It defines the [Intl][Intl]
 class, with the default locale and methods for accessing most of the
 internationalization mechanisms. This library also defines the
 [DateFormat][DateFormat], [NumberFormat][NumberFormat], and
 [BidiFormatter][BidiFormatter] classes.
 
 ## Current locale
 
 The package has a single current locale, called [defaultLocale][defaultLocale].
 Operations will use that locale unless told to do otherwise.
 
-To set the global locale, you can explicitly set it, e.g.
+You can explicitly set the global locale
 
       Intl.defaultLocale = 'pt_BR';
 
-or get it from the browser by
+or get it from the browser
 
       import "package:intl/intl_browser.dart";
       ...
       findSystemLocale().then(runTheRestOfMyProgram);
 
-To temporarily override the current locale, pass the operation
-to [withLocale][withLocale].
+To override the current locale for a particular operation, pass the operation
+to [withLocale][withLocale]. Note that this includes async tasks
+spawned from that operation, and that the argument to
+[withLocale][withLocale]
+will supercede the [defaultLocale][defaultLocale] while the operation
+is active. If you are using different locales within an application,
+the [withLocale][withLocale] operation may be preferable to setting
+[defaultLocale][defaultLocale].
 
       Intl.withLocale('fr', () => print(myLocalizedMessage());
 
-To override it for a very specific operation you can create a format object in
+To specify the locale for an operation you can create a format object in
 a specific locale, or pass in the locale as a parameter to methods.
 
       var format = new DateFormat.yMd("ar");
       var dateString = format.format(new DateTime.now());
       print(myMessage(dateString, locale: 'ar');
 
 ## Initialization
 
 All the different types of locale data require an async initialization step
 to make
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 [NumberFormat]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/intl/intl.NumberFormat
 [withLocale]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/intl/intl.Intl#id_withLocale
 [defaultLocale]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/intl/intl.Intl#id_defaultLocale
 [Intl.message]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/intl/intl.Intl#id_message
 [Intl.plural]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/intl/intl.Intl#id_plural
 [Intl.gender]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/intl/intl.Intl#id_gender
 [DateTime]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/dart:core.DateTime
 [BidiFormatter]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/intl/intl.BidiFormatter
 [BidiFormatter.RTL]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/intl/intl.BidiFormatter#id_BidiFormatter-RTL
 [BidiFormatter.LTR]: https://api.dartlang.org/apidocs/channels/stable/dartdoc-viewer/intl/intl.BidiFormatter#id_BidiFormatter-LTR