Add Null class to dart:core.

sdk/lib/_internal/lib/js_rti.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.
 
 /**
  * This part contains helpers for supporting runtime type information.
  *
  * The helper use a mixture of Dart and JavaScript objects. To indicate which is
  * used where we adopt the scheme of using explicit type annotation for Dart
  * objects and 'var' or omitted return type for JavaScript objects.
@@ skipping 365 matching lines @@
   var typeArguments = getRuntimeTypeArguments(context, contextName);
   return invokeOn(signature, context, typeArguments);
 }
 
 /**
  * Returns [:true:] if the runtime type representation [type] is a supertype of
  * [:Null:].
  */
 bool isSupertypeOfNull(var type) {
   // `null` means `dynamic`.
-  return isNull(type) || getConstructorName(type) == JS_OBJECT_CLASS_NAME();
+  return isNull(type) || getConstructorName(type) == JS_OBJECT_CLASS_NAME()
+                      || getConstructorName(type) == JS_NULL_CLASS_NAME();
 }
 
 /**
  * Tests whether the Dart object [o] is a subtype of the runtime type
  * representation [t].
  *
  * See the comment in the beginning of this file for a description of type
  * representations.
  */
 bool checkSubtypeOfRuntimeType(Object o, var t) {
@@ skipping 310 matching lines @@
  * and [t] are Dart values.
  */
 bool isIdentical(var s, var t) => JS('bool', '# === #', s, t);
 
 /**
  * Returns [:true:] if the JavaScript values [s] and [t] are not identical. We
  * use this helper to avoid generating code under the invalid assumption that
  * [s] and [t] are Dart values.
  */
 bool isNotIdentical(var s, var t) => JS('bool', '# !== #', s, t);