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

runtime/bin/platform_linux.cc

Index: runtime/bin/platform_linux.cc
diff --git a/runtime/bin/platform_linux.cc b/runtime/bin/platform_linux.cc
index 1351c286699e2c4f1e4c7e5276d182bc46ac1e7a..8b114c62cca47ada1769594e92d18803af1b3827 100644
--- a/runtime/bin/platform_linux.cc
+++ b/runtime/bin/platform_linux.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));