Prepare for removal of source_filter.gypi:

runtime/platform/thread_macos.cc

Index: runtime/platform/thread_macos.cc
===================================================================
--- runtime/platform/thread_macos.cc	(revision 18775)
+++ runtime/platform/thread_macos.cc	(working copy)
@@ -2,9 +2,12 @@
 // 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.
 
+#include "platform/globals.h"
+#if defined(TARGET_OS_MACOS)
+
 #include "platform/thread.h"
 
-#include <sys/errno.h>
+#include <sys/errno.h>  // NOLINT
 
 #include "platform/assert.h"
 
@@ -259,3 +262,5 @@
 }
 
 }  // namespace dart
+
+#endif  // defined(TARGET_OS_MACOS)