VM: Remove unnecessary _Num class from runtime/lib.

runtime/lib/num.dart

Index: runtime/lib/num.dart
diff --git a/runtime/lib/num.dart b/runtime/lib/num.dart
deleted file mode 100644
index 7d67213a7dc29e5e446e4fe1e085a79cd9c07f57..0000000000000000000000000000000000000000
--- a/runtime/lib/num.dart
+++ /dev/null
@@ -1,9 +0,0 @@
-// 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";
-
-}