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

sdk/lib/core/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.core;
 
 /**
  * The [Iterable] interface allows to get an [Iterator] out of an
  * [Iterable] object.
  *
@@ skipping 395 matching lines @@
       _index++;
       return true;
     } else {
       _current = null;
       return false;
     }
   }
 
   E get current => _current;
 }
+
+/**
+ * An [Iterator] that allows moving backwards as well as forwards.
+ */
+abstract class BiDirectionalIterator<T> extends Iterator<T> {
+  /**
+   * Move back to the previous element.
+   *
+   * Returns true and updates [current] if successful. Returns false
+   * and sets [current] to null if there is no previous element.
+   */
+  bool movePrevious();
+}