Add additional include guard in platform specific .h files

runtime/platform/thread_linux.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_LINUX_H_
 #define PLATFORM_THREAD_LINUX_H_
 
+#if !defined(PLATFORM_THREAD_H_)
+#error Do not include thread_linux.h directly; use thread.h instead.
+#endif
+
 #include <pthread.h>
 
 #include "platform/assert.h"
 #include "platform/globals.h"
 
 namespace dart {
 
 class ThreadData {
  private:
   ThreadData() {}
@@ skipping 39 matching lines @@
 
   friend class Monitor;
 
   DISALLOW_ALLOCATION();
   DISALLOW_COPY_AND_ASSIGN(MonitorData);
 };
 
 }  // namespace dart
 
 #endif  // PLATFORM_THREAD_LINUX_H_