Make HashSet.length constant time.

sdk/lib/collection/splay_tree.dart

 // Copyright (c) 2012, 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;
 
 /**
  * A node in a splay tree. It holds the sorting key and the left
  * and right children in the tree.
  */
@@ skipping 456 matching lines @@
     _findLeftMostDescendent(_currentNode.right);
     return true;
   }
 
   T _getValue(_SplayTreeNode node);
 }
 
 class _SplayTreeKeyIterable<K> extends Iterable<K> {
   _SplayTree<K> _tree;
   _SplayTreeKeyIterable(this._tree);
+  int get length => _tree._count;
+  bool get isEmpty => _tree._count == 0;
   Iterator<K> get iterator => new _SplayTreeKeyIterator<K>(_tree);
 }
 
 class _SplayTreeValueIterable<K, V> extends Iterable<V> {
   SplayTreeMap<K, V> _map;
   _SplayTreeValueIterable(this._map);
+  int get length => _map._count;
+  bool get isEmpty => _map._count == 0;
   Iterator<V> get iterator => new _SplayTreeValueIterator<K, V>(_map);
 }
 
 class _SplayTreeKeyIterator<K> extends _SplayTreeIterator<K> {
   _SplayTreeKeyIterator(_SplayTree<K> map): super(map);
   K _getValue(_SplayTreeNode node) => node.key;
 }
 
 class _SplayTreeValueIterator<K, V> extends _SplayTreeIterator<V> {
   _SplayTreeValueIterator(SplayTreeMap<K, V> map): super(map);
   V _getValue(_SplayTreeMapNode node) => node.value;
 }
 
 class _SplayTreeNodeIterator<K>
     extends _SplayTreeIterator<_SplayTreeNode<K>> {
   _SplayTreeNodeIterator(_SplayTree<K> map): super(map);
   _SplayTreeNode<K> _getValue(_SplayTreeNode node) => node;
 }