Added localizations for Directionality (#11870)

diff --git a/packages/flutter/lib/src/widgets/app.dart b/packages/flutter/lib/src/widgets/app.dart
index 29962da..a4ec5b5 100644
--- a/packages/flutter/lib/src/widgets/app.dart
+++ b/packages/flutter/lib/src/widgets/app.dart

@@ -28,9 +28,7 @@
   const _WidgetsLocalizationsDelegate();
 
   @override
-  Future<WidgetsLocalizations> load(Locale locale) {
-    return new SynchronousFuture<WidgetsLocalizations>(const WidgetsLocalizations());
-  }
+  Future<WidgetsLocalizations> load(Locale locale) => DefaultWidgetsLocalizations.load(locale);
 
   @override
   bool shouldReload(_WidgetsLocalizationsDelegate old) => false;

diff --git a/packages/flutter/lib/src/widgets/localizations.dart b/packages/flutter/lib/src/widgets/localizations.dart
index 5ed7d60..535d289 100644
--- a/packages/flutter/lib/src/widgets/localizations.dart
+++ b/packages/flutter/lib/src/widgets/localizations.dart

@@ -114,20 +114,13 @@
 /// In particular, this maps locales to a specific [Directionality] using the
 /// [textDirection] property.
 ///
-/// This class provides a default placeholder implementation that returns
-/// hard-coded American English values.
-class WidgetsLocalizations {
-  /// Create a placeholder object for the localized resources of the lowest
-  /// levels of the Flutter framework which only provides values for American
-  /// English.
-  const WidgetsLocalizations();
-
-  /// The locale for which the values of this class's localized resources
-  /// have been translated.
-  Locale get locale => const Locale('en', 'US');
-
+/// See also:
+///
+///  * [DefaultWidgetsLocalizations], which implements this interface and
+///    supports a variety of locales.
+abstract class WidgetsLocalizations {
   /// The reading direction for text in this locale.
-  TextDirection get textDirection => TextDirection.ltr;
+  TextDirection get textDirection;
 
   /// The `WidgetsLocalizations` from the closest [Localizations] instance
   /// that encloses the given context.
@@ -146,6 +139,42 @@
   }
 }
 
+/// Localized values for widgets.
+class DefaultWidgetsLocalizations implements WidgetsLocalizations {
+  DefaultWidgetsLocalizations(this.locale) {
+    final String language = locale.languageCode.toLowerCase();
+    _textDirection = _rtlLanguages.contains(language) ? TextDirection.rtl : TextDirection.ltr;
+  }
+
+  // See http://en.wikipedia.org/wiki/Right-to-left
+  static const List<String> _rtlLanguages = const <String>[
+    'ar',  // Arabic
+    'dv',  // Dhivehi
+    'fa',  // Farsi
+    'he',  // Hebrew
+    'ps',  // Pashto
+    'sd',  // Sindhi
+    'ur',  // Urdu
+  ];
+
+  /// The locale for which the values of this class's localized resources
+  /// have been translated.
+  final Locale locale;
+
+  @override
+  TextDirection get textDirection => _textDirection;
+  TextDirection _textDirection;
+
+  /// Creates an object that provides localized resource values for the
+  /// lowest levels of the Flutter framework.
+  ///
+  /// This method is typically used to create a [LocalizationsDelegate].
+  /// The [WidgetsApp] does so by default.
+  static Future<WidgetsLocalizations> load(Locale locale) {
+    return new SynchronousFuture<WidgetsLocalizations>(new DefaultWidgetsLocalizations(locale));
+  }
+}
+
 class _LocalizationsScope extends InheritedWidget {
   _LocalizationsScope ({
     Key key,

diff --git a/packages/flutter/test/widgets/localizations_test.dart b/packages/flutter/test/widgets/localizations_test.dart
index c08b6ee..18974fa 100644
--- a/packages/flutter/test/widgets/localizations_test.dart
+++ b/packages/flutter/test/widgets/localizations_test.dart

@@ -417,6 +417,26 @@
     expect(modifiedDelegate.shouldReloadValues, <bool>[true]);
   });
 
+  testWidgets('Directionality tracks system locale', (WidgetTester tester) async {
+    BuildContext pageContext;
+
+    await tester.pumpWidget(
+      buildFrame(
+        buildContent: (BuildContext context) {
+          pageContext = context;
+          return const Text('Hello World');
+        }
+      )
+    );
+
+    await tester.binding.setLocale('en', 'GB');
+    await tester.pump();
+    expect(Directionality.of(pageContext), TextDirection.ltr);
+
+    await tester.binding.setLocale('ar', 'EG');
+    await tester.pump();
+    expect(Directionality.of(pageContext), TextDirection.rtl);
+  });
 }
 
 // Same as _WidgetsLocalizationsDelegate in widgets/app.dart
@@ -424,9 +444,7 @@
   const DefaultWidgetsLocalizationsDelegate();
 
   @override
-  Future<WidgetsLocalizations> load(Locale locale) {
-    return new SynchronousFuture<WidgetsLocalizations>(const WidgetsLocalizations());
-  }
+  Future<WidgetsLocalizations> load(Locale locale) => DefaultWidgetsLocalizations.load(locale);
 
   @override
   bool shouldReload(DefaultWidgetsLocalizationsDelegate old) => false;