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

runtime/vm/isolate_macos.cc

Index: runtime/vm/isolate_macos.cc
diff --git a/runtime/vm/isolate_macos.cc b/runtime/vm/isolate_macos.cc
index 5ebf08000f561ea2cc5010fd68ec205b5bfd019e..ac5432fdc34c0713e6e119d5ffd8fb97bdb91243 100644
--- a/runtime/vm/isolate_macos.cc
+++ b/runtime/vm/isolate_macos.cc
@@ -1,12 +1,13 @@
-// 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/isolate.h"
+
 #include <errno.h>
 #include <pthread.h>
 
-#include "vm/assert.h"
-#include "vm/isolate.h"
+#include "platform/assert.h"
 
 namespace dart {