Make HashSet.length constant time.

sdk/lib/collection/linked_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 LinkedHashSet<E> extends Collection<E> implements Set<E> {
   static const int _INITIAL_CAPACITY = 8;
   _LinkedHashTable<E> _table;
 
@@ skipping 12 matching lines @@
     int offset = _table._next(_LinkedHashTable._HEAD_OFFSET);
     int modificationCount = _table._modificationCount;
     while (offset != _LinkedHashTable._HEAD_OFFSET) {
       E key = _table._key(offset);
       action(key);
       _table._checkModification(modificationCount);
       offset = _table._next(offset);
     }
   }
 
+  int get length => _table._elementCount;
+
   bool get isEmpty => _table._elementCount == 0;
 
   bool contains(Object object) => _table._get(object) >= 0;
 
   E get first {
     int firstOffset = _table._next(_LinkedHashTable._HEAD_OFFSET);
     if (firstOffset == _LinkedHashTable._HEAD_OFFSET) {
       throw new StateError("No elements");
     }
     return _table._key(firstOffset);
@@ skipping 105 matching lines @@
     for (E element in other) {
       if (this.contains(element)) {
         result.add(element);
       }
     }
     return result;
   }
 
   String toString() => Collections.collectionToString(this);
 }