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

runtime/vm/os_macos.cc

Index: runtime/vm/os_macos.cc
diff --git a/runtime/vm/os_macos.cc b/runtime/vm/os_macos.cc
index 0f1d68f087ecd6a492364139814145305bb4f709..8f4be1b19f01f313e9e39305f4376b461819967c 100644
--- a/runtime/vm/os_macos.cc
+++ b/runtime/vm/os_macos.cc
@@ -1,7 +1,9 @@
-// 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 "vm/os.h"
+
 #include <errno.h>
 #include <limits.h>
 #include <mach/mach.h>
@@ -12,7 +14,6 @@
 #include <unistd.h>
 
 #include "vm/isolate.h"
-#include "vm/os.h"
 #include "vm/utils.h"
 
 namespace dart {