Add Null class to dart:core.

sdk/lib/core/null.dart

+// 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";
+}