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

runtime/vm/os_thread_macos.cc

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 #include "platform/globals.h"  // NOLINT
 #if defined(TARGET_OS_MACOS)
 
 #include "vm/os_thread.h"
 
 #include <sys/errno.h>  // NOLINT
@@ skipping 104 matching lines @@
   result = pthread_create(&tid, &attr, ThreadStart, data);
   RETURN_ON_PTHREAD_FAILURE(result);
 
   result = pthread_attr_destroy(&attr);
   RETURN_ON_PTHREAD_FAILURE(result);
 
   return 0;
 }
 
 
-ThreadLocalKey OSThread::kUnsetThreadLocalKey =
-    static_cast<pthread_key_t>(-1);
-ThreadId OSThread::kInvalidThreadId = reinterpret_cast<ThreadId>(NULL);
-ThreadJoinId OSThread::kInvalidThreadJoinId =
+const ThreadId OSThread::kInvalidThreadId = reinterpret_cast<ThreadId>(NULL);
+const ThreadJoinId OSThread::kInvalidThreadJoinId =
     reinterpret_cast<ThreadJoinId>(NULL);
 
+
 ThreadLocalKey OSThread::CreateThreadLocal(ThreadDestructor destructor) {
   pthread_key_t key = kUnsetThreadLocalKey;
   int result = pthread_key_create(&key, destructor);
   VALIDATE_PTHREAD_RESULT(result);
   ASSERT(key != kUnsetThreadLocalKey);
   return key;
 }
 
 
 void OSThread::DeleteThreadLocal(ThreadLocalKey key) {
@@ skipping 276 matching lines @@
 void Monitor::NotifyAll() {
   // When running with assertions enabled we track the owner.
   ASSERT(IsOwnedByCurrentThread());
   int result = pthread_cond_broadcast(data_.cond());
   VALIDATE_PTHREAD_RESULT(result);
 }
 
 }  // namespace dart
 
 #endif  // defined(TARGET_OS_MACOS)