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.
+ *
+ * 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";
+}