Make sure that Dart_Ports are printed safely in the service protocol

runtime/vm/message.cc

Index: runtime/vm/message.cc
diff --git a/runtime/vm/message.cc b/runtime/vm/message.cc
index dc638b2602cdf8d902671a584a31e6b3baca99d0..54f40b4eeb77c91cee4a35bbd2c5d356a0e75b48 100644
--- a/runtime/vm/message.cc
+++ b/runtime/vm/message.cc
@@ -196,8 +196,8 @@ void MessageQueue::PrintJSON(JSONStream* stream) {
                          current->Id());
     message.AddProperty("size", current->len());
     message.AddProperty("index", depth++);
-    message.AddProperty("_destinationPort",
-        static_cast<intptr_t>(current->dest_port()));
+    message.AddPropertyF("_destinationPort", "%" Pd64 "",
+        static_cast<int64_t>(current->dest_port()));
     message.AddProperty("_priority",
         Message::PriorityAsString(current->priority()));
     // TODO(johnmccutchan): Move port -> handler map out of Dart and into the