Extend Iterable instead of implementing it.

sdk/lib/_internal/compiler/implementation/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.
 
 part of org_dartlang_compiler_util;
 
-class Link<T> implements Iterable<T> {
+class Link<T> extends Iterable<T> {
   T get head => null;
   Link<T> get tail => null;
 
   factory Link.fromList(List<T> list) {
     switch (list.length) {
       case 0:
         return new Link<T>();
       case 1:
         return new LinkEntry<T>(list[0]);
       case 2:
@@ skipping 43 matching lines @@
 }
 
 interface LinkBuilder<T> default LinkBuilderImplementation<T> {
   LinkBuilder();
 
   Link<T> toLink();
   void addLast(T t);
 
   final int length;
 }