Add reduce to Collection.

corelib/src/collection.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.
 
 /**
  * The [Collection] interface is the public interface of all
  * collections.
  */
 interface Collection<E> extends Iterable<E> {
   /**
    * Applies the function [f] to each element of this collection.
    */
   void forEach(void f(E element));
 
   /**
    * Returns a new collection with the elements [: f(e) :]
-   * for each element [e] of this collection. 
+   * for each element [:e:] of this collection.
    *
    * Note on typing: the return type of f() could be an arbitrary
    * type and consequently the returned collection's
-   * typeis Collection.  
+   * typeis Collection.
    */
   Collection map(f(E element));
 
   /**
+   * Reduce a collection to a single value by iteratively combining each element
+   * of the collection with an existing value using the provided function.
+   * Use [initialValue] as the initial value, and the function [combine] to
+   * create a new value from the previous one and an element.
+   *
+   * Example of calculating the sum of a collection:
+   *
+   *   collection.reduce(0, (prev, element) => prev + element);
+   */
+  Dynamic reduce(Dynamic initialValue,
+                 Dynamic combine(Dynamic previousValue, E element));
+
+  /**
    * Returns a new collection with the elements of this collection
    * that satisfy the predicate [f].
    *
    * An element satisfies the predicate [f] if [:f(element):]
    * returns true.
    */
   Collection<E> filter(bool f(E element));
 
   /**
    * Returns true if every elements of this collection satisify the
@@ skipping 10 matching lines @@
   /**
    * Returns true if there is no element in this collection.
    */
   bool isEmpty();
 
   /**
    * Returns the number of elements in this collection.
    */
   int get length();
 }