Don't try to send messages, when the port is not set.

runtime/bin/eventhandler_win.cc

Index: runtime/bin/eventhandler_win.cc
diff --git a/runtime/bin/eventhandler_win.cc b/runtime/bin/eventhandler_win.cc
index fbc562398d297c730c1c9b84532b92653fae4465..3b12d940847b641f4714086de811f645bd2da436 100644
--- a/runtime/bin/eventhandler_win.cc
+++ b/runtime/bin/eventhandler_win.cc
@@ -344,8 +344,10 @@ static void HandleError(Handle* handle) {
   handle->set_last_error(WSAGetLastError());
   handle->MarkError();
   if (!handle->IsClosing()) {
-    int event_mask = 1 << kErrorEvent;
-    DartUtils::PostInt32(handle->port(), event_mask);
+    Dart_Port port = handle->port();
+    if (port != ILLEGAL_PORT) {
+      DartUtils::PostInt32(port, 1 << kErrorEvent);
+    }
   }
 }