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

runtime/bin/platform_macos.cc

Index: runtime/bin/platform_macos.cc
diff --git a/runtime/bin/platform_macos.cc b/runtime/bin/platform_macos.cc
index b2182496bcd3ba72b5967339bc5cc6c056f0fec5..84cce9293ea7a5951b3d8f83bd54f40409721a9e 100644
--- a/runtime/bin/platform_macos.cc
+++ b/runtime/bin/platform_macos.cc
@@ -34,6 +34,11 @@ const char* Platform::OperatingSystem() {
 }
 
 
+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));