Revert "Remaining private libs"

sdk/lib/collection/hash_set.dart

 // Copyright (c) 2013, 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.
 
-part of "dart:collection";
+part of "collection.dart";
 
 /** Common parts of [HashSet] and [LinkedHashSet] implementations. */
 abstract class _HashSetBase<E> extends SetBase<E> {
   // The following two methods override the ones in SetBase.
   // It's possible to be more efficient if we have a way to create an empty
   // set of the correct type.
 
   Set<E> difference(Set<Object> other) {
     Set<E> result = _newSet();
     for (var element in this) {
@@ skipping 120 matching lines @@
   }
 
   /**
    * Provides an iterator that iterates over the elements of this set.
    *
    * The order of iteration is unspecified,
    * but consistent between changes to the set.
    */
   Iterator<E> get iterator;
 }