Make Null a public class of dart:core.

runtime/lib/null_patch.dart

+// Copyright (c) 2013, 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.
+
+patch class Null {
+
+  factory Null._uninstantiable() {
+    throw new UnsupportedError(
+        "class Null cannot be instantiated");
+  }
+
+  int get hashCode {
+    return 2011;  // The year Dart was announced and a prime.
+  }
+
+  String toString() {
+    return 'null';
+  }
+}