Uses an open thread handle as the ThreadJoinId on Windows.

runtime/vm/thread_pool.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 VM_THREAD_POOL_H_
 #define VM_THREAD_POOL_H_
 
 #include "vm/allocation.h"
 #include "vm/globals.h"
 #include "vm/os_thread.h"
@@ skipping 107 matching lines @@
 
   bool RemoveWorkerFromIdleList(Worker* worker);
   bool RemoveWorkerFromAllList(Worker* worker);
 
   void AddWorkerToShutdownList(Worker* worker);
   bool RemoveWorkerFromShutdownList(Worker* worker);
 
   void ReapExitedIdleThreads();
 
   // Worker operations.
+  void SetIdleLocked(Worker* worker);  // Assumes mutex_ is held.
   void SetIdleAndReapExited(Worker* worker);
   bool ReleaseIdleWorker(Worker* worker);
 
   Mutex mutex_;
   bool shutting_down_;
   Worker* all_workers_;
   Worker* idle_workers_;
   uint64_t count_started_;
   uint64_t count_stopped_;
   uint64_t count_running_;
   uint64_t count_idle_;
 
   Monitor exit_monitor_;
   Worker* shutting_down_workers_;
   JoinList* join_list_;
 
   DISALLOW_COPY_AND_ASSIGN(ThreadPool);
 };
 
 }  // namespace dart
 
 #endif  // VM_THREAD_POOL_H_