Update or delete language tests using obsolete interfaces or default factory

tests/language/library12.dart

Index: tests/language/library12.dart
===================================================================
--- tests/language/library12.dart	(revision 15648)
+++ tests/language/library12.dart	(working copy)
@@ -1,4 +1,4 @@
-// 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.
 //
@@ -33,7 +33,7 @@
   static var static_fld = 4;
 }
 
-interface Library12Interface {
+abstract class Library12Interface {
   Library12 addObjects(Library12 value1, Library12 value2);
 }