Revert various changes to Link factories.

dart/frog/leg/util/link.dart

Index: dart/frog/leg/util/link.dart
diff --git a/dart/frog/leg/util/link.dart b/dart/frog/leg/util/link.dart
index 26e7fa4e8fdaca044e4e933092ecd17021dbebb9..4a61cb51c2912d146929984b9aa3274deab17e36 100644
--- a/dart/frog/leg/util/link.dart
+++ b/dart/frog/leg/util/link.dart
@@ -2,7 +2,7 @@
 // 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.
 
-interface Link<T> extends Iterable<T> {
+interface Link<T> extends Iterable<T> factory LinkFactory {
   final T head;
   final Link<T> tail;