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

runtime/vm/thread_linux.cc

Index: runtime/vm/thread_linux.cc
diff --git a/runtime/vm/thread_linux.cc b/runtime/vm/thread_linux.cc
index faf445fba7013fac0125427467f2eac249aedb90..e537eb6e66df1bc10708217ffbac51b2528aa931 100644
--- a/runtime/vm/thread_linux.cc
+++ b/runtime/vm/thread_linux.cc
@@ -1,13 +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/thread.h"
+
 #include <errno.h>
 #include <sys/time.h>
 
-#include "vm/thread.h"
-
-#include "vm/assert.h"
+#include "platform/assert.h"
 
 namespace dart {