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

runtime/vm/os_thread_win.h

Index: runtime/vm/os_thread_win.h
diff --git a/runtime/vm/os_thread_win.h b/runtime/vm/os_thread_win.h
index 539818282c5a2f497c6f5cb4e4ecc8c8830db155..790306dd59b6d2a11ba3e1edd51abedfec3fe832 100644
--- a/runtime/vm/os_thread_win.h
+++ b/runtime/vm/os_thread_win.h
@@ -20,13 +20,16 @@ typedef DWORD ThreadLocalKey;
 typedef DWORD ThreadId;
 typedef DWORD ThreadJoinId;
 
+
+static const ThreadLocalKey kUnsetThreadLocalKey = TLS_OUT_OF_INDEXES;
+
+
 class ThreadInlineImpl {
  private:
   ThreadInlineImpl() {}
   ~ThreadInlineImpl() {}
 
   static uword GetThreadLocal(ThreadLocalKey key) {
-    static ThreadLocalKey kUnsetThreadLocalKey = TLS_OUT_OF_INDEXES;
     ASSERT(key != kUnsetThreadLocalKey);
     return reinterpret_cast<uword>(TlsGetValue(key));
   }