Revert various changes to Link factories.

dart/frog/leg/util/link.dart

 // Copyright (c) 2011, 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.
 
-interface Link<T> extends Iterable<T> {
+interface Link<T> extends Iterable<T> factory LinkFactory {
   final T head;
   final Link<T> tail;
 
   Link(T head, [Link<T> tail]);
   Link.fromList(List<T> list);
 
   Link<T> prepend(T element);
   List<T> toList();
   bool isEmpty();
   Link<T> reverse();
 
   void printOn(StringBuffer buffer, [separatedBy]);
 }
 
 interface EmptyLink<T> extends Link<T> factory LinkTail {
   const EmptyLink();
 }
 
 interface LinkBuilder<T> factory LinkBuilderImplementation {
   LinkBuilder();
 
   Link<T> toLink();
   void addLast(T t);
 }