Add reduce to Collection.

corelib/src/collection.dart

Index: corelib/src/collection.dart
diff --git a/corelib/src/collection.dart b/corelib/src/collection.dart
index 719145acab615314900df2e1bd675bf6b97b6340..b6f656a635194edd69dd55aae5c50e27023e316a 100644
--- a/corelib/src/collection.dart
+++ b/corelib/src/collection.dart
@@ -14,15 +14,28 @@ interface Collection<E> extends Iterable<E> {
 
   /**
    * 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].
    *