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

runtime/platform/thread_macos.h

 // 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.
 
 #ifndef PLATFORM_THREAD_MACOS_H_
 #define PLATFORM_THREAD_MACOS_H_
 
 #if !defined(PLATFORM_THREAD_H_)
 #error Do not include thread_macos.h directly; use thread.h instead.
 #endif
 
 #include <pthread.h>
 
 #include "platform/globals.h"
 
 namespace dart {
 
+typedef pthread_key_t ThreadLocalKey;
+
 class MutexData {
  private:
   MutexData() {}
   ~MutexData() {}
 
   pthread_mutex_t* mutex() { return &mutex_; }
 
   pthread_mutex_t mutex_;
 
   friend class Mutex;
@@ skipping 16 matching lines @@
 
   friend class Monitor;
 
   DISALLOW_ALLOCATION();
   DISALLOW_COPY_AND_ASSIGN(MonitorData);
 };
 
 }  // namespace dart
 
 #endif  // PLATFORM_THREAD_MACOS_H_