Extend dart:io error handling to all socket functions

runtime/bin/utils_macos.cc

Index: runtime/bin/utils_macos.cc
diff --git a/runtime/bin/utils_macos.cc b/runtime/bin/utils_macos.cc
index 0b4a69fe35bf6d8295046b8f8c963c1d234cc8ad..f364e475700fc02c3e01b21471720b03858b6afb 100644
--- a/runtime/bin/utils_macos.cc
+++ b/runtime/bin/utils_macos.cc
@@ -3,11 +3,26 @@
 // BSD-style license that can be found in the LICENSE file.
 
 #include <errno.h>
+#include <netdb.h>
 
 #include "bin/utils.h"
+#include "platform/assert.h"
 
 OSError::OSError() : sub_system_(kSystem), code_(0), message_(NULL) {
   set_sub_system(kSystem);
   set_code(errno);
   SetMessage(strerror(errno));
 }
+
+
+void OSError::SetCodeAndMessage(SubSystem sub_system, int code) {
+  set_sub_system(sub_system);
+  set_code(code);
+  if (sub_system == kSystem) {
+    SetMessage(strerror(code));
+  } else if (sub_system == kGetAddressInfo) {
+    SetMessage(gai_strerror(code));
+  } else {
+    UNREACHABLE();
+  }
+}