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

runtime/vm/os_win.cc

Index: runtime/vm/os_win.cc
diff --git a/runtime/vm/os_win.cc b/runtime/vm/os_win.cc
index 70d239cf172c8de558aa374f4748daed8a5c3660..2acef13fe304b92e735082d851fb144e2524bd51 100644
--- a/runtime/vm/os_win.cc
+++ b/runtime/vm/os_win.cc
@@ -1,11 +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 "vm/os.h"
+
 #include <time.h>
 
-#include "vm/assert.h"
-#include "vm/os.h"
+#include "platform/assert.h"
 
 namespace dart {