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 {

[Lasse Reichstein Nielsen, 2013/08/16 08:04:13]

Add /* patch */ in front?

+    return 2011;  // The year Dart was announced and a prime.

[Lasse Reichstein Nielsen, 2013/08/16 08:04:13]

I'd prefer either zero or some real random smi.
"Arbitrary" hash codes like this tend to cluster in the low end of the range.

I can see this was just retained from earlier, but do consider updating it. We
could add it to the Null class directly, if dart2js sees no problem with it.

+  }
+
+  String toString() {
+    return 'null';

[Lasse Reichstein Nielsen, 2013/08/16 08:04:13]

Both the factory constructor and the toString methods are already in the Null
class. No need to override them.

+  }
+}