Make HashSet.length constant time.

sdk/lib/collection/splay_tree.dart

Index: sdk/lib/collection/splay_tree.dart
diff --git a/sdk/lib/collection/splay_tree.dart b/sdk/lib/collection/splay_tree.dart
index 4e8362f7c38bf200a0235f92db5a1130ca811b1a..bd2361d1b1c5b82e31a85e6f75fa31ec94f3124b 100644
--- a/sdk/lib/collection/splay_tree.dart
+++ b/sdk/lib/collection/splay_tree.dart
@@ -474,12 +474,16 @@ abstract class _SplayTreeIterator<T> implements Iterator<T> {
 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);
 }