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..506d6e7965615c7e0fdbe74ff468ce269730b1be 100644
--- a/runtime/vm/os_thread.h
+++ b/runtime/vm/os_thread.h
@@ -67,7 +67,7 @@ class OSThread : public BaseThread {
   }
 
   ThreadId trace_id() const {
-    ASSERT(trace_id_ != OSThread::kInvalidThreadJoinId);
+    ASSERT(trace_id_ != OSThread::kInvalidThreadId);
     return trace_id_;
   }
 
@@ -205,7 +205,11 @@ class OSThread : public BaseThread {
 
   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);