VM thread shutdown.

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 36436aae58dd1a80fa08b9033f73f7112103c890..30ac50966d1ef7268280769bdca08ef533a2f820 100644
--- a/runtime/vm/os_thread.h
+++ b/runtime/vm/os_thread.h
@@ -29,6 +29,7 @@ class OSThread {
  public:
   static ThreadLocalKey kUnsetThreadLocalKey;
   static ThreadId kInvalidThreadId;
+  static ThreadJoinId kInvalidThreadJoinId;
 
   typedef void (*ThreadStartFunction) (uword parameter);
   typedef void (*ThreadDestructor) (void* parameter);
@@ -47,7 +48,8 @@ class OSThread {
   static void SetThreadLocal(ThreadLocalKey key, uword value);
   static intptr_t GetMaxStackSize();
   static ThreadId GetCurrentThreadId();
-  static bool Join(ThreadId id);
+  static ThreadJoinId GetCurrentThreadJoinId();
+  static void Join(ThreadJoinId id);
   static intptr_t ThreadIdToIntPtr(ThreadId id);
   static ThreadId ThreadIdFromIntPtr(intptr_t id);
   static bool Compare(ThreadId a, ThreadId b);