[dart:io] Adds Platform.localeName

tests/standalone/io/locale_name_test.dart

+// Copyright (c) 2017, 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.
+
+import 'dart:io';
+
+import "package:expect/expect.dart";
+
+main() {
+  try {
+    Platform.localeName;
+  } catch (e, s) {
+    Expect.fail("Platform.localeName threw: $e\n$s\n");
+  }
+  Expect.isNotNull(Platform.localeName);
+  Expect.isTrue(Platform.localeName is String);
+  print(Platform.localeName);
+}