Allow private selectors to be typed, and fix ClassElement.isSubclassOf to take patches into account.

tests/language/private_selector_test.dart

Index: tests/language/private_selector_test.dart
===================================================================
--- tests/language/private_selector_test.dart	(revision 0)
+++ tests/language/private_selector_test.dart	(revision 0)
@@ -0,0 +1,15 @@
+// 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 private_selector_test;
+
+import 'private_selector_lib.dart';
+
+class B extends A {
+}
+
+main() {
+  new A().public();
+  Expect.isTrue(executed);
+}