Uses an open thread handle as the ThreadJoinId on Windows.

runtime/vm/os_thread.cc

Index: runtime/vm/os_thread.cc
diff --git a/runtime/vm/os_thread.cc b/runtime/vm/os_thread.cc
index 947b7f6955f318d3eef1b39aec80ad2285a38e4f..75a7a31850d3b201fc552bd87481a8e9cd3e208d 100644
--- a/runtime/vm/os_thread.cc
+++ b/runtime/vm/os_thread.cc
@@ -23,7 +23,9 @@ bool OSThread::creation_enabled_ = false;
 OSThread::OSThread() :
     BaseThread(true),
     id_(OSThread::GetCurrentThreadId()),
-    join_id_(OSThread::GetCurrentThreadJoinId()),
+#if defined(DEBUG)
+    join_id_(kInvalidThreadJoinId),
+#endif
     trace_id_(OSThread::GetCurrentThreadTraceId()),
     name_(NULL),
     timeline_block_lock_(new Mutex()),
@@ -160,8 +162,8 @@ OSThread* OSThread::CreateAndSetUnknownThread() {
 }
 
 
-bool OSThread::IsThreadInList(ThreadJoinId join_id) {
-  if (join_id == OSThread::kInvalidThreadJoinId) {
+bool OSThread::IsThreadInList(ThreadId id) {
+  if (id == OSThread::kInvalidThreadId) {
     return false;
   }
   OSThreadIterator it;
@@ -169,8 +171,8 @@ bool OSThread::IsThreadInList(ThreadJoinId join_id) {
     ASSERT(OSThread::thread_list_lock_->IsOwnedByCurrentThread());
     OSThread* t = it.Next();
     // An address test is not sufficient because the allocator may recycle
-    // the address for another Thread. Test against the thread's join id.
-    if (t->join_id() == join_id) {
+    // the address for another Thread. Test against the thread's id.
+    if (t->id() == id) {
       return true;
     }
   }