Make Null a public class of dart:core.

runtime/lib/null_patch.dart

Index: runtime/lib/null_patch.dart
diff --git a/runtime/lib/mirror_reference.dart b/runtime/lib/null_patch.dart
similarity index 50%
copy from runtime/lib/mirror_reference.dart
copy to runtime/lib/null_patch.dart
index b0f7726d7fbeb8548a4a2ee8aa9a24e3e1834e19..07eb99f0280d7efc78577de0d8dde8412b883212 100644
--- a/runtime/lib/mirror_reference.dart
+++ b/runtime/lib/null_patch.dart
@@ -2,11 +2,20 @@
 // 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.
 
-class _MirrorReference {
-  factory _MirrorReference._uninstantiable() {
+// Dart core library.
+
+class Null {
+
+  factory Null._uninstantiable() {
     throw new UnsupportedError(
-        "class _MirrorReference cannot be instantiated");
+        "class Null cannot be instantiated");
   }
 
-  bool operator ==(other) native "MirrorReference_equals";
+  int get hashCode {
+    return 2011;  // The year Dart was announced and a prime.
+  }
+
+  String toString() {
+    return 'null';
+  }
 }