Uses an open thread handle as the ThreadJoinId on Windows.

runtime/bin/thread_macos.cc

Index: runtime/bin/thread_macos.cc
diff --git a/runtime/bin/thread_macos.cc b/runtime/bin/thread_macos.cc
index 8286835897438152876daeb8432bd409c3e7c1f8..9477e8ab4a834322439b87c6cbb2b9dc59e3110e 100644
--- a/runtime/bin/thread_macos.cc
+++ b/runtime/bin/thread_macos.cc
@@ -148,11 +148,6 @@ ThreadId Thread::GetCurrentThreadId() {
 }
 
 
-bool Thread::Join(ThreadId id) {
-  return false;
-}
-
-
 intptr_t Thread::ThreadIdToIntPtr(ThreadId id) {
   ASSERT(sizeof(id) == sizeof(intptr_t));
   return reinterpret_cast<intptr_t>(id);