Make Null a public class of dart:core.

runtime/lib/null_patch.dart

Index: runtime/lib/null_patch.dart
diff --git a/runtime/lib/null_patch.dart b/runtime/lib/null_patch.dart
new file mode 100644
index 0000000000000000000000000000000000000000..cfc1f33bfb0465a2a0e3a8df914f50794e8f67c1
--- /dev/null
+++ b/runtime/lib/null_patch.dart
@@ -0,0 +1,21 @@
+// 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';
+  }
+}