Add Iterable.fold (and Stream.fold) which replace `reduce`.

sdk/lib/collection/collections.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.
 
 part of dart.collection;
 
 /**
  * This class provides default implementations for Iterables (including Lists).
  *
  * Once Dart receives Mixins it will be replaced with mixin classes.
@@ skipping 22 matching lines @@
   static bool every(Iterable iterable, bool f(o)) {
     for (final e in iterable) {
       if (!f(e)) return false;
     }
     return true;
   }
 
   static dynamic reduce(Iterable iterable,
                         dynamic initialValue,
                         dynamic combine(dynamic previousValue, element)) {
+    return fold(iterable, initialValue, combine);
+  }
+
+  static dynamic fold(Iterable iterable,
+                      dynamic initialValue,
+                      dynamic combine(dynamic previousValue, element)) {
     for (final element in iterable) {
       initialValue = combine(initialValue, element);
     }
     return initialValue;
   }
 
   /**
    * Simple implementation for [Collection.removeAll].
    *
    * This implementation assumes that [Collection.remove] on [collection]
@@ skipping 369 matching lines @@
  * The source of the elements may be a [List] or any [Iterable] with
  * efficient [Iterable.length] and [Iterable.elementAt].
  */
 class UnmodifiableListView<E> extends UnmodifiableListBase<E> {
   Iterable<E> _source;
   /** Create an unmodifiable list backed by [source]. */
   UnmodifiableListView(Iterable<E> source) : _source = source;
   int get length => _source.length;
   E operator[](int index) => _source.elementAt(index);
 }