Add member localHostname() to the Dart IO Platform class.

runtime/bin/platform_impl.dart

 // Copyright (c) 2011, 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.
 
 class _Platform implements Platform {
   _Platform();
 
   static int _numberOfProcessors() native "Platform_NumberOfProcessors";
   static String _pathSeparator() native "Platform_PathSeparator";
   static String _operatingSystem() native "Platform_OperatingSystem";
+  static _localHostname() native "Platform_LocalHostname";
 
   int numberOfProcessors() {
     return _numberOfProcessors();
   }
 
   String pathSeparator() {
     return _pathSeparator();
   }
 
   String operatingSystem() {
     return _operatingSystem();
   }
+
+  String localHostname() {
+    var result = _localHostname();
+    if (result is OSError) {
+      throw result;
+    } else {
+      return result;
+    }
+  }
 }