Add reduce to Collection.

corelib/src/implementation/hash_map_set.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.
 
 // Hash map implementation with open addressing and quadratic probing.
 class HashMapImplementation<K extends Hashable, V> implements HashMap<K, V> {
 
   // The [_keys] list contains the keys inserted in the map.
   // The [_keys] list must be a raw list because it
   // will contain both elements of type K, and the [_DELETED_KEY] of type
@@ skipping 332 matching lines @@
   }
 
   Set map(f(E element)) {
     Set result = new Set();
     _backingMap.forEach(void _(E key, E value) {
       result.add(f(key));
     });
     return result;
   }
 
+  Dynamic reduce(Dynamic initialValue,
+                 Dynamic combine(Dynamic previousValue, E element)) {
+    return Collections.reduce(this, initialValue, combine);
+  }
+
   Set<E> filter(bool f(E element)) {
     Set<E> result = new Set<E>();
     _backingMap.forEach(void _(E key, E value) {
       if (f(key)) result.add(key);
     });
     return result;
   }
 
   bool every(bool f(E element)) {
     Collection<E> keys = _backingMap.getKeys();
@@ skipping 77 matching lines @@
 
 /**
  * A singleton sentinel used to represent when a key is deleted from the map.
  * We can't use [: const Object() :] as a sentinel because it would end up
  * canonicalized and then we cannot distinguish the deleted key from the
  * canonicalized [: Object() :].
  */
 class _DeletedKeySentinel {
   const _DeletedKeySentinel();
 }