Add Null class to dart:core.

sdk/lib/core/null.dart

Index: sdk/lib/core/null.dart
diff --git a/sdk/lib/core/null.dart b/sdk/lib/core/null.dart
new file mode 100644
index 0000000000000000000000000000000000000000..46960e01fb3513774809b15710e6c85b024189d4
--- /dev/null
+++ b/sdk/lib/core/null.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.
+
+part of dart.core;
+
+/**
+ * Class of the `null` value.
+ *
+ * The only instance of the [Null] class is the `null` object.
+ *
+ * It is an error for a class to extend or implement Null.
+ */
+class Null {
+  factory Null._uninstantiable() {
+    throw new UnsupportedError('class Null cannot be instantiated');
+  }
+
+  /** Returns the string `"null"`. */
+  String toString() => "null";
+}