Reapply 'Add reduce to Collection.'

corelib/src/implementation/hash_map_set.dart

Index: corelib/src/implementation/hash_map_set.dart
diff --git a/corelib/src/implementation/hash_map_set.dart b/corelib/src/implementation/hash_map_set.dart
index c538951fff31beb62674103b8c7517339c0c8ded..c3d45790e5f4af5bb17c6c62442fd8e765b925c0 100644
--- a/corelib/src/implementation/hash_map_set.dart
+++ b/corelib/src/implementation/hash_map_set.dart
@@ -350,6 +350,11 @@ class HashSetImplementation<E extends Hashable> implements HashSet<E> {
     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) {