Add support for building the Dart VM for Android OS.

runtime/platform/thread_android.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_
+#ifndef PLATFORM_THREAD_ANDROID_H_
+#define PLATFORM_THREAD_ANDROID_H_
 
 #if !defined(PLATFORM_THREAD_H_)
-#error Do not include thread_macos.h directly; use thread.h instead.
+#error Do not include thread_android.h directly; use thread.h instead.
 #endif
 
 #include <pthread.h>
 
 #include "platform/assert.h"
 #include "platform/globals.h"
 
 namespace dart {
 
 typedef pthread_key_t ThreadLocalKey;
@@ skipping 44 matching lines @@
   pthread_cond_t cond_;
 
   friend class Monitor;
 
   DISALLOW_ALLOCATION();
   DISALLOW_COPY_AND_ASSIGN(MonitorData);
 };
 
 }  // namespace dart
 
-#endif  // PLATFORM_THREAD_MACOS_H_
+#endif  // PLATFORM_THREAD_ANDROID_H_