Add mirrors tests for Null type.

tests/lib/mirrors/relation_assignable_test.dart

 // Copyright (c) 2014, 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.
 
 library test.relation_assignable;
 
 import "dart:mirrors";
 
 import "package:expect/expect.dart";
 
@@ skipping 15 matching lines @@
 
 test(MirrorSystem mirrors) {
   LibraryMirror coreLibrary = mirrors.findLibrary(#dart.core);
   LibraryMirror thisLibrary = mirrors.findLibrary(#test.relation_assignable);
 
   // Classes.
   TypeMirror Super = thisLibrary.declarations[#Superclass];
   TypeMirror Sub1 = thisLibrary.declarations[#Subclass1];
   TypeMirror Sub2 = thisLibrary.declarations[#Subclass2];
   TypeMirror Obj = coreLibrary.declarations[#Object];
+  TypeMirror Nul = coreLibrary.declarations[#Null];
 
   Expect.isTrue(Obj.isAssignableTo(Obj));
   Expect.isTrue(Super.isAssignableTo(Super));
   Expect.isTrue(Sub1.isAssignableTo(Sub1));
   Expect.isTrue(Sub2.isAssignableTo(Sub2));
+  Expect.isTrue(Nul.isAssignableTo(Nul));
 
   Expect.isTrue(Sub1.isAssignableTo(Super));
   Expect.isTrue(Super.isAssignableTo(Sub1));
 
   Expect.isTrue(Sub2.isAssignableTo(Super));
   Expect.isTrue(Super.isAssignableTo(Sub2));
 
   Expect.isFalse(Sub2.isAssignableTo(Sub1));
   Expect.isFalse(Sub1.isAssignableTo(Sub2));
 
   Expect.isTrue(Sub1.isAssignableTo(Obj));
   Expect.isTrue(Obj.isAssignableTo(Sub1));
 
   Expect.isTrue(Sub2.isAssignableTo(Obj));
   Expect.isTrue(Obj.isAssignableTo(Sub2));
 
   Expect.isTrue(Super.isAssignableTo(Obj));
   Expect.isTrue(Obj.isAssignableTo(Super));
 
+  Expect.isTrue(Nul.isAssignableTo(Obj));
+  Expect.isTrue(Obj.isAssignableTo(Nul));
+  Expect.isTrue(Nul.isAssignableTo(Super));  // Null type is bottom type.
+  Expect.isTrue(Super.isAssignableTo(Nul));
 
   // Function typedef - argument type.
   TypeMirror Func = coreLibrary.declarations[#Function];
   TypedefMirror NumPred = thisLibrary.declarations[#NumberPredicate];
   TypedefMirror IntPred = thisLibrary.declarations[#IntegerPredicate];
   TypedefMirror DubPred = thisLibrary.declarations[#DoublePredicate];
 
   Expect.isTrue(Func.isAssignableTo(Func));
   Expect.isTrue(NumPred.isAssignableTo(NumPred));
   Expect.isTrue(IntPred.isAssignableTo(IntPred));
@@ skipping 240 matching lines @@
   Expect.isFalse(Void.isAssignableTo(TFromB));
   Expect.isFalse(Void.isAssignableTo(TFromC));
 
   Expect.isTrue(Dynamic.isAssignableTo(Void));
   Expect.isTrue(Void.isAssignableTo(Dynamic));
 }
 
 main() {
   test(currentMirrorSystem());
 }