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

runtime/bin/platform_impl.dart

Index: runtime/bin/platform_impl.dart
diff --git a/runtime/bin/platform_impl.dart b/runtime/bin/platform_impl.dart
index 0328e65dca6eee36f748af7597958aa8147edea1..f859854e89cdeca9e3b20a6841ce45b30197b8ad 100644
--- a/runtime/bin/platform_impl.dart
+++ b/runtime/bin/platform_impl.dart
@@ -8,6 +8,7 @@ class _Platform implements 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();
@@ -20,4 +21,13 @@ class _Platform implements Platform {
   String operatingSystem() {
     return _operatingSystem();
   }
+
+  String localHostname() {
+    var result = _localHostname();
+    if (result is OSError) {
+      throw result;
+    } else {
+      return result;
+    }
+  }
 }