Reapply 'Add reduce to Collection.'

corelib/src/implementation/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.
 
 /**
  * The [Collections] class implements static methods useful when
  * writing a class that implements [Collection] and the [iterator]
  * method.
  */
 class Collections {
@@ skipping 17 matching lines @@
     return true;
   }
 
   static List map(Iterable source, List destination, f(o)) {
     for (final e in source) {
       destination.add(f(e));
     }
     return destination;
   }
 
+  static Dynamic reduce(Iterable iterable,
+                        Dynamic initialValue,
+                        Dynamic combine(Dynamic previousValue, element)) {
+    for (final element in iterable) {
+      initialValue = combine(initialValue, element);
+    }
+    return initialValue;
+  }
+
   static List filter(Iterable source, List destination, bool f(o)) {
     for (final e in source) {
       if (f(e)) destination.add(e);
     }
     return destination;
   }
 
   static bool isEmpty(Iterable iterable) {
     return !iterable.iterator().hasNext();
   }
@@ skipping 89 matching lines @@
    * Returns true if the specified collection contains the specified object
    * reference.
    */
   static _containsRef(Collection c, Object ref) {
     for (var e in c) {
       if (e === ref) return true;
     }
     return false;
   }
 }