Avoid UTF-8 decoding in num.toString.

runtime/lib/num_patch.dart

Index: runtime/lib/num_patch.dart
===================================================================
--- runtime/lib/num_patch.dart	(revision 0)
+++ runtime/lib/num_patch.dart	(revision 0)
@@ -0,0 +1,12 @@
+// 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.
+// Dart core library.
+
+// VM implementation of num.
+
+patch class num {
+
+  /* patch */ String toString() native "num_toString";
+
+}