Uses an open thread handle as the ThreadJoinId on Windows.

runtime/vm/os_thread.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_OS_THREAD_H_
 #define VM_OS_THREAD_H_
 
 #include "platform/globals.h"
 #include "vm/allocation.h"
 #include "vm/globals.h"
@@ skipping 49 matching lines @@
     ASSERT(id_ != OSThread::kInvalidThreadId);
     return id_;
   }
 
   ThreadJoinId join_id() const {
     ASSERT(join_id_ != OSThread::kInvalidThreadJoinId);
     return join_id_;
   }
 
   ThreadId trace_id() const {
-    ASSERT(trace_id_ != OSThread::kInvalidThreadJoinId);
+    ASSERT(trace_id_ != OSThread::kInvalidThreadId);
     return trace_id_;
   }
 
   const char* name() const {
     return name_;
   }
 
   void SetName(const char* name);
 
   void set_name(const char* name) {
@@ skipping 117 matching lines @@
   // in the windows thread interrupter which is used for profiling.
   // We could eliminate this requirement if the windows thread interrupter
   // is implemented differently.
   Thread* thread() const { return thread_; }
   void set_thread(Thread* value) {
     thread_ = value;
   }
 
   static void Cleanup();
   static ThreadId GetCurrentThreadTraceId();
+
+  // WARNING: The Windows implementation relies on this function begin called
+  // only once per-thread in the constructor for an OSThread.

[Ivan Posva, 2016/05/16 17:13:14]

We should enforce this as we discussed instead of relying on the comment here.
The failure mode is pretty bad as far as I can tell.

   static ThreadJoinId GetCurrentThreadJoinId();
+
   static OSThread* GetOSThreadFromThread(Thread* thread);
   static void AddThreadToListLocked(OSThread* thread);
   static void RemoveThreadFromList(OSThread* thread);
   static OSThread* CreateAndSetUnknownThread();
 
   static ThreadLocalKey thread_key_;
 
   const ThreadId id_;
   const ThreadJoinId join_id_;
   const ThreadId trace_id_;  // Used to interface with tracing tools.
@@ skipping 125 matching lines @@
   friend class SafepointMonitorLocker;
   friend void Dart_TestMonitor();
   DISALLOW_COPY_AND_ASSIGN(Monitor);
 };
 
 
 }  // namespace dart
 
 
 #endif  // VM_OS_THREAD_H_