Runes, a bi-directional code-point iterator/iterable.

sdk/lib/_collection_dev/iterable.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 dart._collection.dev;
 
 typedef T _Transformation<S, T>(S value);
 
 class MappedIterable<S, T> extends Iterable<T> {
   final Iterable<S> _iterable;
@@ skipping 665 matching lines @@
 
   Set toSet() => new Set<E>();
 }
 
 /** The always empty iterator. */
 class EmptyIterator<E> implements Iterator<E> {
   const EmptyIterator();
   bool moveNext() => false;
   E get current => null;
 }
+
+/** An [Iterator] that can move in both directions. */
+abstract class BiDirectionalIterator<T> implements Iterator<T> {
+  bool movePrevious();
+}