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

runtime/bin/platform_macos.cc

 // 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.
 
 #include "bin/platform.h"
 
 #include <signal.h>
 #include <string.h>
 #include <unistd.h>
 
@@ skipping 16 matching lines @@
 int Platform::NumberOfProcessors() {
   return sysconf(_SC_NPROCESSORS_ONLN);
 }
 
 
 const char* Platform::OperatingSystem() {
   return "macos";
 }
 
 
+bool Platform::LocalHostname(char *buffer, intptr_t buffer_length) {
+  return gethostname(buffer, buffer_length) == 0;
+}
+
+
 char* Platform::StrError(int error_code) {
   static const int kBufferSize = 1024;
   char* error = static_cast<char*>(malloc(kBufferSize));
   error[0] = '\0';
   strerror_r(error_code, error, kBufferSize);
   return error;
 }