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

runtime/bin/platform.cc

Index: runtime/bin/platform.cc
diff --git a/runtime/bin/platform.cc b/runtime/bin/platform.cc
index 97c1a6a91f40ca76caad58f8b95a211bb18971b3..f8398f415174719fb6ffcb594c9d64983281b8b0 100644
--- a/runtime/bin/platform.cc
+++ b/runtime/bin/platform.cc
@@ -26,3 +26,16 @@ void FUNCTION_NAME(Platform_PathSeparator)(Dart_NativeArguments args) {
   Dart_SetReturnValue(args, Dart_NewString(File::PathSeparator()));
   Dart_ExitScope();
 }
+
+
+void FUNCTION_NAME(Platform_LocalHostname)(Dart_NativeArguments args) {
+  Dart_EnterScope();
+  const intptr_t HOSTNAME_LENGTH = 256;
+  char hostname[HOSTNAME_LENGTH];
+  if (Platform::LocalHostname(hostname, HOSTNAME_LENGTH)) {
+    Dart_SetReturnValue(args, Dart_NewString(hostname));
+  } else {
+    Dart_SetReturnValue(args, DartUtils::NewDartOSError());
+  }
+  Dart_ExitScope();
+}