Update or delete language tests using obsolete interfaces or default factory

tests/language/generic_deep_test.dart

Index: tests/language/generic_deep_test.dart
===================================================================
--- tests/language/generic_deep_test.dart	(revision 15648)
+++ tests/language/generic_deep_test.dart	(working copy)
@@ -1,11 +1,11 @@
-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
+// Copyright (c) 2012, 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.
 
 // Dart test for deeply nested generic types.
 
 /** A natural number aka Peano number. */
-interface N {
+abstract class N {
   N add1();
   N sub1();
 }