Add mirrors tests for Null type.

tests/lib/mirrors/relation_subclass_test.dart

Index: tests/lib/mirrors/relation_subclass_test.dart
diff --git a/tests/lib/mirrors/relation_subclass_test.dart b/tests/lib/mirrors/relation_subclass_test.dart
index 2e8bbc24793d60b9e35945b73fc041ec7da894b7..c0c79273d361909abacabb8675a3b1a7cf60779b 100644
--- a/tests/lib/mirrors/relation_subclass_test.dart
+++ b/tests/lib/mirrors/relation_subclass_test.dart
@@ -28,11 +28,13 @@ test(MirrorSystem mirrors) {
   ClassMirror Sub1 = thisLibrary.declarations[#Subclass1];
   ClassMirror Sub2 = thisLibrary.declarations[#Subclass2];
   ClassMirror Obj = coreLibrary.declarations[#Object];
+  ClassMirror Nul = coreLibrary.declarations[#Null];
 
   Expect.isTrue(Obj.isSubclassOf(Obj));
   Expect.isTrue(Super.isSubclassOf(Super));
   Expect.isTrue(Sub1.isSubclassOf(Sub1));
   Expect.isTrue(Sub2.isSubclassOf(Sub2));
+  Expect.isTrue(Nul.isSubclassOf(Nul));
 
   Expect.isTrue(Sub1.isSubclassOf(Super));
   Expect.isFalse(Super.isSubclassOf(Sub1));
@@ -52,6 +54,11 @@ test(MirrorSystem mirrors) {
   Expect.isTrue(Super.isSubclassOf(Obj));
   Expect.isFalse(Obj.isSubclassOf(Super));
 
+  Expect.isTrue(Nul.isSubclassOf(Obj));
+  Expect.isFalse(Obj.isSubclassOf(Nul));
+  Expect.isFalse(Nul.isSubclassOf(Super));
+  Expect.isFalse(Super.isSubclassOf(Nul));
+
   var Func = coreLibrary.declarations[#Function];
   Expect.isTrue(Func.isSubclassOf(Obj));
   Expect.isFalse(Obj.isSubclassOf(Func));