Fix null.runtimeType, reflect(null).type.superinterfaces, and reflect(null).getField.

tests/lib/mirrors/null2_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';
 
-class Foo {
+main() {
+  InstanceMirror nullMirror = reflect(null);
+  for (int i = 0; i < 10; i++) {
+    Expect.isTrue(nullMirror.getField(#hashCode).reflectee is int);
+  }
 }
-
-main() {
-  Expect.stringEquals('Foo', '${new Foo().runtimeType}');
-  Expect.stringEquals('foo', MirrorSystem.getName(new Symbol('foo')));
-}