Avoid UTF-8 decoding in num.toString.

runtime/lib/num.dart

Index: runtime/lib/num.dart
===================================================================
--- runtime/lib/num.dart	(revision 0)
+++ runtime/lib/num.dart	(revision 0)
@@ -0,0 +1,9 @@
+// Copyright (c) 2014, 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.
+
+abstract class _Num implements num {
+
+  /* patch */ String toString() native "Num_toString";
+
+}