Uses an open thread handle as the ThreadJoinId on Windows.

runtime/vm/os_thread.h

Index: runtime/vm/os_thread.h
diff --git a/runtime/vm/os_thread.h b/runtime/vm/os_thread.h
index 1bd2193b20cbd3ca951e522836c52e6836f22b1a..415cff89b5dcceb9719b616355f60e3461c89b9a 100644
--- a/runtime/vm/os_thread.h
+++ b/runtime/vm/os_thread.h
@@ -61,13 +61,8 @@ class OSThread : public BaseThread {
     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_;
   }
 
@@ -174,10 +169,14 @@ class OSThread : public BaseThread {
   static ThreadId ThreadIdFromIntPtr(intptr_t id);
   static bool Compare(ThreadId a, ThreadId b);
 
+  // This function can be called only once per OSThread, and should only be
+  // called when the retunred id will eventually be passed to OSThread::Join().
+  static ThreadJoinId GetCurrentThreadJoinId(OSThread* thread);
+
   // Called at VM startup and shutdown.
   static void InitOnce();
 
-  static bool IsThreadInList(ThreadJoinId join_id);
+  static bool IsThreadInList(ThreadId id);
 
   static void DisableOSThreadCreation();
   static void EnableOSThreadCreation();
@@ -205,7 +204,6 @@ class OSThread : public BaseThread {
 
   static void Cleanup();
   static ThreadId GetCurrentThreadTraceId();
-  static ThreadJoinId GetCurrentThreadJoinId();
   static OSThread* GetOSThreadFromThread(Thread* thread);
   static void AddThreadToListLocked(OSThread* thread);
   static void RemoveThreadFromList(OSThread* thread);
@@ -214,7 +212,11 @@ class OSThread : public BaseThread {
   static ThreadLocalKey thread_key_;
 
   const ThreadId id_;
-  const ThreadJoinId join_id_;
+#if defined(DEBUG)
+  // In DEBUG mode we use this field to ensure that GetCurrentThreadJoinId is
+  // only called once per OSThread.
+  ThreadJoinId join_id_;
+#endif
   const ThreadId trace_id_;  // Used to interface with tracing tools.
   char* name_;  // A name for this thread.