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

[Søren Gjesse, 2013/08/13 14:53:52]

Comment cut short.

[rmacnak, 2013/08/14 01:23:13]

"It is a compile-time error for a class to attempt to extend or implement Null."

[Lasse Reichstein Nielsen, 2013/08/14 08:18:35]

Done.

+ */
+class Null {
+  /**

[Søren Gjesse, 2013/08/13 14:53:52]

Will this dartdoc ever end up in the documentation as _ is private?

[Lasse Reichstein Nielsen, 2013/08/14 08:18:35]

Probably, but not displayed by default.
I'll copy the bool constructor and omit the comment.

+   * Instantiating `Null` is not allowed.
+   *
+   * This constructor is only here to prevent the automatic
+   * generation of a default constructor.
+   */
+  factory Null._() {

[rmacnak, 2013/08/14 01:23:13]

Please mimic the factory in bool.

[Lasse Reichstein Nielsen, 2013/08/14 08:18:35]

Done.

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