Make HashSet.length constant time.

sdk/lib/_collection_dev/iterable.dart

 // Copyright (c) 2011, 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.dev;
 
 /**
  * An [Iterable] for classes that have efficient [length] and [elementAt].
  *
  * All other methods are implemented in terms of [length] and [elementAt],
@@ skipping 10 matching lines @@
   void forEach(void action(E element)) {
     int length = this.length;
     for (int i = 0; i < length; i++) {
       action(elementAt(i));
       if (length != this.length) {
         throw new ConcurrentModificationError(this);
       }
     }
   }
 
-  bool get isEmpty => length != 0;
+  bool get isEmpty => length == 0;
 
   E get first {
     if (length == 0) throw new StateError("No elements");
     return elementAt(0);
   }
 
   E get last {
     if (length == 0) throw new StateError("No elements");
     return elementAt(length - 1);
   }
@@ skipping 648 matching lines @@
 class EmptyIterator<E> implements Iterator<E> {
   const EmptyIterator();
   bool moveNext() => false;
   E get current => null;
 }
 
 /** An [Iterator] that can move in both directions. */
 abstract class BidirectionalIterator<T> implements Iterator<T> {
   bool movePrevious();
 }