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

runtime/vm/os_thread_macos.cc

Index: runtime/vm/os_thread_macos.cc
diff --git a/runtime/vm/os_thread_macos.cc b/runtime/vm/os_thread_macos.cc
index db9dfeb960479095101d58ba0483ad6a781e89c7..2fa58d85456e73334b5501e2b501079ca4070e90 100644
--- a/runtime/vm/os_thread_macos.cc
+++ b/runtime/vm/os_thread_macos.cc
@@ -122,12 +122,11 @@ int OSThread::Start(const char* name,
 }
 
 
-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);