Make HashSet.length constant time.

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;
 
 class HashSet<E> extends Collection<E> implements Set<E> {
   static const int _INITIAL_CAPACITY = 8;
   final _HashTable<E> _table;
 
   HashSet() : _table = new _HashTable(_INITIAL_CAPACITY) {
     _table._container = this;
   }
 
   factory HashSet.from(Iterable<E> iterable) {
     return new HashSet<E>()..addAll(iterable);
   }
 
   // Iterable.
   Iterator<E> get iterator => new _HashTableKeyIterator<E>(_table);
 
+  int get length => _table._elementCount;
+
   bool get isEmpty => _table._elementCount == 0;
 
   bool contains(Object object) => _table._get(object) >= 0;
 
   // Collection.
   void add(E element) {
     _table._put(element);
     _table._checkCapacity();
   }
 
@@ skipping 74 matching lines @@
     for (E element in other) {
       if (this.contains(element)) {
         result.add(element);
       }
     }
     return result;
   }
 
   String toString() =>  Collections.collectionToString(this);
 }