[dart:io] Reland: Adds Platform.localeName

sdk/lib/io/platform_impl.dart

Index: sdk/lib/io/platform_impl.dart
diff --git a/sdk/lib/io/platform_impl.dart b/sdk/lib/io/platform_impl.dart
index 5f68b7dbdef6ded8dc6ddaf263e8090d5ae94326..e5de17d02e52d6d18fe867ad5b0ac19a0c035c63 100644
--- a/sdk/lib/io/platform_impl.dart
+++ b/sdk/lib/io/platform_impl.dart
@@ -32,12 +32,25 @@ class _Platform {
   external static String _packageRoot();
   external static String _packageConfig();
   external static String _version();
+  external static String _localeName();
 
   static String executable = _executable();
   static String resolvedExecutable = _resolvedExecutable();
   static String packageRoot = _packageRoot();
   static String packageConfig = _packageConfig();
 
+  static String _cachedLocaleName;
+  static String get localeName {
+    if (_cachedLocaleName == null) {
+      var result = _localeName();
+      if (result is OSError) {
+        throw result;
+      }
+      _cachedLocaleName = result;
+    }
+    return _cachedLocaleName;
+  }
+
   // Cache the OS environment. This can be an OSError instance if
   // retrieving the environment failed.
   static var /*OSError|Map<String,String>*/ _environmentCache;