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

sdk/lib/core/iterable.dart

Index: sdk/lib/core/iterable.dart
diff --git a/sdk/lib/core/iterable.dart b/sdk/lib/core/iterable.dart
index fc1a647b179d3217474f2578b147ea0fa92d099b..56e96ef048a5b7a1e99480a160b1cffd0614978c 100644
--- a/sdk/lib/core/iterable.dart
+++ b/sdk/lib/core/iterable.dart
@@ -432,3 +432,16 @@ class _GeneratorIterator<E> implements Iterator<E> {
 
   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();
+}