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

runtime/bin/eventhandler_win.cc

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/globals.h"
 #if defined(TARGET_OS_WINDOWS)
 
 #include "bin/eventhandler.h"
 
 #include <winsock2.h>  // NOLINT
@@ skipping 326 matching lines @@
     int event_mask = 1 << kCloseEvent;
     DartUtils::PostInt32(handle->port(), event_mask);
   }
 }
 
 
 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);
+    }
   }
 }
 
 
 void Handle::HandleIssueError() {
   DWORD error = GetLastError();
   if (error == ERROR_BROKEN_PIPE) {
     HandleClosed(this);
   } else {
     HandleError(this);
@@ skipping 987 matching lines @@
 
 
 void EventHandlerImplementation::Shutdown() {
   SendData(kShutdownId, 0, 0);
 }
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_WINDOWS)