Make Null a public class of dart:core.

tests/language/null_is_test.dart

 // 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.
 
 import "package:expect/expect.dart";
 
 main() {
   Expect.isTrue(null is Object);
+  Expect.isTrue(null is Null);
   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);
+
+  test(null);
+
+  Expect.isFalse(1 is Null);
+  Expect.isFalse("1" is Null);
+  Expect.isFalse(true is Null);
+  Expect.isFalse(false is Null);
+  Expect.isFalse(new Object() is Null);
+
+  testNegative(1);
+  testNegative("1");
+  testNegative(true);
+  testNegative(false);
+  testNegative(new Object());
 }
+
+test(n) {
+  // Test where the argument is not a compile-time constant.
+  Expect.isTrue(n is Object);
+  Expect.isTrue(n is Null);
+  Expect.isFalse(n is int);
+  Expect.isFalse(n is bool);
+  Expect.isFalse(n is num);
+  Expect.isFalse(n is String);
+  Expect.isFalse(n is List);
+  Expect.isFalse(n is Expect);
+}
+
+testNegative(n){
+        Expect.isFalse(n is Null);
+}