Avoid UTF-8 decoding in num.toString.

runtime/lib/num.cc

Index: runtime/lib/num.cc
===================================================================
--- runtime/lib/num.cc	(revision 0)
+++ runtime/lib/num.cc	(revision 0)
@@ -0,0 +1,17 @@
+// 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.
+
+#include "vm/bootstrap_natives.h"
+
+#include "vm/native_entry.h"
+#include "vm/object.h"
+
+namespace dart {
+
+DEFINE_NATIVE_ENTRY(Num_toString, 1) {
+  const Number& number = Number::CheckedHandle(arguments->NativeArgAt(0));
+  return number.ToString(Heap::kNew);
+}
+
+}  // namespace dart