Fix double-closing of handles in Windows EventHandler.

runtime/bin/eventhandler_win.h

 // Copyright (c) 2012, 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.
 
 #ifndef BIN_EVENTHANDLER_WIN_H_
 #define BIN_EVENTHANDLER_WIN_H_
 
 #if !defined(BIN_EVENTHANDLER_H_)
 #error Do not include eventhandler_win.h directly; use eventhandler.h instead.
 #endif
@@ skipping 216 matching lines @@
 
   void Lock();
   void Unlock();
 
   bool CreateCompletionPort(HANDLE completion_port);
 
   void Close();
   virtual void DoClose();
   virtual bool IsClosed() = 0;
 
+  bool IsHandleClosed() const { return handle_ == INVALID_HANDLE_VALUE; }
+
   void SetPortAndMask(Dart_Port port, intptr_t mask) {
     port_ = port;
     mask_ = mask;
   }
   Type type() { return type_; }
   bool is_file() { return type_ == kFile; }
   bool is_socket() { return type_ == kListenSocket ||
                             type_ == kClientSocket ||
                             type_ == kDatagramSocket; }
   bool is_listen_socket() { return type_ == kListenSocket; }
@@ skipping 284 matching lines @@
 
   TimeoutQueue timeout_queue_;  // Time for next timeout.
   bool shutdown_;
   HANDLE completion_port_;
 };
 
 }  // namespace bin
 }  // namespace dart
 
 #endif  // BIN_EVENTHANDLER_WIN_H_