Add helpers library to dart2js.

sdk/lib/_internal/compiler/implementation/helpers/expensive_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 dart2js.util;
+part of dart2js.helpers;
 
 /**
  * The expensive set is a data structure useful for tracking down
  * excessive memory usage due to large sets. It acts as an ordinary
  * hash set, but it uses 10 times more memory (by default).
  */
 class ExpensiveSet<E> extends IterableBase<E> implements Set<E> {
 
   final List _sets;
 
@@ skipping 99 matching lines @@
     if (objectsToRetain is Set) {
       retainSet = objectsToRetain;
     } else {
       retainSet = objectsToRetain.toSet();
     }
     retainWhere(retainSet.contains);
   }
 
   String toString() => "expensive(${_sets[0]}x${_sets.length})";
 }