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..fb8d5290e62292a0c5f8b358aeb57f6a5274058f
--- /dev/null
+++ b/sdk/lib/core/null.dart
@@ -0,0 +1,26 @@
+// 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.
+ *
+ * Extending or implementing `Null` is not allowed, and the
+ */
+class Null {
+  /**
+   * Instantiating `Null` is not allowed.
+   *
+   * This factory constructor always throws.
+   */
+  factory Null() {

[Lasse Reichstein Nielsen, 2013/08/13 08:25:58]

Should this be a private constructor, e.g., "Null._".
It is only here to prevent the default constructor from being added, not because
anyone needs to call it.

+    throw new UnsupportedError('new Null()');
+  }
+
+  /** Returns the string `"null"`. */
+  String toString() => "null";
+}