Remove analyzer warning from test.

tests/language/factory5_test.dart

 // 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.
 
 abstract class Link<T> {
   factory Link.create() = LinkFactory<T>.create;
 }
 
-class LinkFactory<T> {
+class LinkFactory<T> implements Link<T> {
   factory LinkFactory.create() { return null; }
 }
 
 main() {
   var a = new Link<int>.create();
 }