Make Null a public class of dart:core.

tests/lib/mirrors/null_test.dart

 // Copyright (c) 2013, 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.null_test;
 
 import 'dart:mirrors';
 
 import 'package:expect/expect.dart';
 
 main() {
   InstanceMirror nullMirror = reflect(null);
   Expect.isTrue(nullMirror.getField(const Symbol('hashCode')).reflectee is int);
   Expect.equals(null.hashCode,
                 nullMirror.getField(const Symbol('hashCode')).reflectee);
   Expect.equals('Null',
                 nullMirror.getField(const Symbol('runtimeType')).reflectee
                 .toString());
   Expect.isTrue(nullMirror.invoke(const Symbol('=='), [null]).reflectee);
   Expect.isFalse(nullMirror.invoke(const Symbol('=='), [new Object()])
                  .reflectee);
   Expect.equals('null',
                 nullMirror.invoke(const Symbol('toString'), []).reflectee);
   Expect.throws(() => nullMirror.invoke(const Symbol('notDefined'), []),
                 (e) => e is NoSuchMethodError,
                 'noSuchMethod');
 
   ClassMirror NullMirror = nullMirror.type;
+  Expect.equals(reflectClass(Null), NullMirror);
   Expect.equals(const Symbol('Null'), NullMirror.simpleName);
   Expect.equals(const Symbol('Object'), NullMirror.superclass.simpleName);
   Expect.equals(null, NullMirror.superclass.superclass);
   Expect.listEquals([], NullMirror.superinterfaces);
   Expect.equals(currentMirrorSystem().libraries[Uri.parse('dart:core')],
                 NullMirror.owner);
 }