Revert "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 c3d45790e5f4af5bb17c6c62442fd8e765b925c0..c538951fff31beb62674103b8c7517339c0c8ded 100644
--- a/corelib/src/implementation/hash_map_set.dart
+++ b/corelib/src/implementation/hash_map_set.dart
@@ -350,11 +350,6 @@ 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) {