Add mirrors tests for Null type.

tests/lib/mirrors/relation_subtype_test.dart

Index: tests/lib/mirrors/relation_subtype_test.dart
diff --git a/tests/lib/mirrors/relation_subtype_test.dart b/tests/lib/mirrors/relation_subtype_test.dart
index 9504c8cc4ed7bc1b487f8315047b53447d5eeb0b..ab015ce03c83e914b56515dac6a1233cde491f26 100644
--- a/tests/lib/mirrors/relation_subtype_test.dart
+++ b/tests/lib/mirrors/relation_subtype_test.dart
@@ -33,11 +33,13 @@ test(MirrorSystem mirrors) {
   TypeMirror Sub1 = thisLibrary.declarations[#Subclass1];
   TypeMirror Sub2 = thisLibrary.declarations[#Subclass2];
   TypeMirror Obj = coreLibrary.declarations[#Object];
+  TypeMirror Nul = coreLibrary.declarations[#Null];
 
   Expect.isTrue(Obj.isSubtypeOf(Obj));
   Expect.isTrue(Super.isSubtypeOf(Super));
   Expect.isTrue(Sub1.isSubtypeOf(Sub1));
   Expect.isTrue(Sub2.isSubtypeOf(Sub2));
+  Expect.isTrue(Nul.isSubtypeOf(Nul));
 
   Expect.isTrue(Sub1.isSubtypeOf(Super));
   Expect.isFalse(Super.isSubtypeOf(Sub1));
@@ -57,6 +59,10 @@ test(MirrorSystem mirrors) {
   Expect.isTrue(Super.isSubtypeOf(Obj));
   Expect.isFalse(Obj.isSubtypeOf(Super));
 
+  Expect.isTrue(Nul.isSubtypeOf(Obj));
+  Expect.isFalse(Obj.isSubtypeOf(Nul));
+  Expect.isTrue(Nul.isSubtypeOf(Super));  // Null type is bottom type.
+  Expect.isFalse(Super.isSubtypeOf(Nul));
 
   // Function typedef - argument type.
   TypeMirror Func = coreLibrary.declarations[#Function];