Extend Iterable instead of implementing it.

sdk/lib/_internal/compiler/implementation/mirrors/mirrors_util.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.
 
 library mirrors_util;
 
 // TODO(rnystrom): Use "package:" URL (#4968).
 import 'mirrors.dart';
 
 //------------------------------------------------------------------------------
@@ skipping 35 matching lines @@
 LibraryMirror findLibrary(MemberMirror member) {
   DeclarationMirror owner = member.owner;
   if (owner is LibraryMirror) {
     return owner;
   } else if (owner is TypeMirror) {
     return owner.library;
   }
   throw new Exception('Unexpected owner: ${owner}');
 }
 
-class HierarchyIterable implements Iterable<ClassMirror> {
+class HierarchyIterable extends Iterable<ClassMirror> {
   final bool includeType;
   final ClassMirror type;
 
   HierarchyIterable(this.type, {bool includeType})
       : this.includeType = includeType;
 
   Iterator<ClassMirror> iterator() =>
       new HierarchyIterator(type, includeType: includeType);
 }
 
@@ skipping 39 matching lines @@
       type = object;
       object = null;
       return type;
     } else {
       return push(queue.removeFirst());
     }
   }
 
   bool get hasNext => !queue.isEmpty || object != null;
 }