Fix 'is' for null.

dart/tests/language/src/NullIsTest.dart

Index: dart/tests/language/src/NullIsTest.dart
===================================================================
--- dart/tests/language/src/NullIsTest.dart	(revision 0)
+++ dart/tests/language/src/NullIsTest.dart	(revision 0)
@@ -0,0 +1,13 @@
+// Copyright (c) 2012, 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.
+
+main() {
+  Expect.isTrue(null is Object);
+  Expect.isFalse(null is int);
+  Expect.isFalse(null is bool);
+  Expect.isFalse(null is num);
+  Expect.isFalse(null is String);
+  Expect.isFalse(null is List);
+  Expect.isFalse(null is Expect);
+}