Automatic thread cleanup on non-Windows platforms.

runtime/vm/os_thread.h

Index: runtime/vm/os_thread.h
===================================================================
--- runtime/vm/os_thread.h	(revision 45808)
+++ runtime/vm/os_thread.h	(working copy)
@@ -30,6 +30,7 @@
   static ThreadId kInvalidThreadId;
 
   typedef void (*ThreadStartFunction) (uword parameter);
+  typedef void (*ThreadDestructor) (void* parameter);
 
   // Start a thread running the specified function. Returns 0 if the
   // thread started successfuly and a system specific error code if
@@ -36,7 +37,8 @@
   // the thread failed to start.
   static int Start(ThreadStartFunction function, uword parameters);
 
-  static ThreadLocalKey CreateThreadLocal();
+  // NOTE: Destructor currently ignored on Windows (issue 23474).
+  static ThreadLocalKey CreateThreadLocal(ThreadDestructor destructor = NULL);
   static void DeleteThreadLocal(ThreadLocalKey key);
   static uword GetThreadLocal(ThreadLocalKey key) {
     return ThreadInlineImpl::GetThreadLocal(key);