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

runtime/vm/os_thread_linux.cc

Index: runtime/vm/os_thread_linux.cc
diff --git a/runtime/vm/os_thread_linux.cc b/runtime/vm/os_thread_linux.cc
index 2bf570902fd05657895de38beb6355911221b838..4a08b84d2377ef98e2670088e5d5251bac760f31 100644
--- a/runtime/vm/os_thread_linux.cc
+++ b/runtime/vm/os_thread_linux.cc
@@ -131,10 +131,10 @@ int OSThread::Start(const char* name,
 }
 
 
-ThreadLocalKey OSThread::kUnsetThreadLocalKey =
-    static_cast<pthread_key_t>(-1);
-ThreadId OSThread::kInvalidThreadId = static_cast<ThreadId>(0);
-ThreadJoinId OSThread::kInvalidThreadJoinId = static_cast<ThreadJoinId>(0);
+const ThreadId OSThread::kInvalidThreadId = static_cast<ThreadId>(0);
+const ThreadJoinId OSThread::kInvalidThreadJoinId =
+    static_cast<ThreadJoinId>(0);
+
 
 ThreadLocalKey OSThread::CreateThreadLocal(ThreadDestructor destructor) {
   pthread_key_t key = kUnsetThreadLocalKey;