Make Null a public class of dart:core.

tests/language/null_is_test.dart

Index: tests/language/null_is_test.dart
diff --git a/tests/language/null_is_test.dart b/tests/language/null_is_test.dart
index 395d47495ac1f4843ca3be1441c49b30d4666c50..380dbce91ffb3e1691cae8b466095cb5253c1d1a 100644
--- a/tests/language/null_is_test.dart
+++ b/tests/language/null_is_test.dart
@@ -6,10 +6,41 @@ 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);
 }