Move assert.h/assert.cc from runtime/vm to runtime/platform

runtime/vm/thread_macos.cc

Index: runtime/vm/thread_macos.cc
diff --git a/runtime/vm/thread_macos.cc b/runtime/vm/thread_macos.cc
index 97e09e746d4c754fc99df3522ab802993fbcdda4..b9a8a1d7d6cefa23694a4aebde8be83e1bc059c5 100644
--- a/runtime/vm/thread_macos.cc
+++ b/runtime/vm/thread_macos.cc
@@ -1,12 +1,12 @@
-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// 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.
 
-#include <sys/errno.h>
-
 #include "vm/thread.h"
 
-#include "vm/assert.h"
+#include <sys/errno.h>
+
+#include "platform/assert.h"
 
 namespace dart {