VM: Small clean up and const-ness fix for Thread/OSThread constants.

runtime/vm/os_thread_win.cc

Index: runtime/vm/os_thread_win.cc
diff --git a/runtime/vm/os_thread_win.cc b/runtime/vm/os_thread_win.cc
index c46405ae1f68c69edb74ad35e9af59f0b59b2b4b..aac5639a064647f4e24f4149572b2a8ab4af053e 100644
--- a/runtime/vm/os_thread_win.cc
+++ b/runtime/vm/os_thread_win.cc
@@ -86,9 +86,10 @@ int OSThread::Start(const char* name,
   return 0;
 }
 
-ThreadLocalKey OSThread::kUnsetThreadLocalKey = TLS_OUT_OF_INDEXES;
-ThreadId OSThread::kInvalidThreadId = 0;
-ThreadJoinId OSThread::kInvalidThreadJoinId = 0;
+
+const ThreadId OSThread::kInvalidThreadId = 0;
+const ThreadJoinId OSThread::kInvalidThreadJoinId = 0;
+
 
 ThreadLocalKey OSThread::CreateThreadLocal(ThreadDestructor destructor) {
   ThreadLocalKey key = TlsAlloc();