Move the thread local functions to the Thread class in runtime/platform

runtime/vm/isolate_macos.h

-// Copyright (c) 2011, 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.
-
-#ifndef VM_ISOLATE_MACOS_H_
-#define VM_ISOLATE_MACOS_H_
-
-#if !defined(VM_ISOLATE_H_)
-#error Do not include isolate_macos.h directly; use isolate.h instead.
-#endif
-
-#include <pthread.h>
-
-namespace dart {
-
-#define PTHREAD_KEY_UNSET static_cast<pthread_key_t>(-1)
-extern pthread_key_t isolate_key;
-
-
-inline Isolate* Isolate::Current() {
-  ASSERT(isolate_key != PTHREAD_KEY_UNSET);
-  return reinterpret_cast<Isolate*>(pthread_getspecific(isolate_key));
-}
-
-}  // namespace dart
-
-#endif  // VM_ISOLATE_MACOS_H_