Change Set.toSet to always return a set with the same behavior.

pkg/collection/lib/wrappers.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.
 
 /**
  * Delegating wrappers for [Iterable], [List], [Set], [Queue] and [Map].
  *
  * Also adds unmodifiable views for `Set` and `Map`, and a fixed length
  * view for `List`. The unmodifable list view from `dart:collection` is exported
  * as well, just for completeness.
@@ skipping 224 matching lines @@
 
   void retainAll(Iterable<Object> elements) {
     _setBase.retainAll(elements);
   }
 
   void retainWhere(bool test(E element)) {
     _setBase.retainWhere(test);
   }
 
   Set<E> union(Set<E> other) => _setBase.union(other);
+
+  Set<E> toSet() => new DelegatingSet<E>(_setBase.toSet());
+
+  Set<E> cloneEmpty() => new DelegatingSet<E>(_setBase.cloneEmpty());
 }
 
 /**
  * Creates a [Queue] that delegates all operations to a base queue.
  *
  * This class can be used hide non-`Queue` methods of a queue object,
  * or it can be extended to add extra functionality on top of an existing
  * queue object.
  */
 class DelegatingQueue<E> extends DelegatingIterable<E> implements Queue<E> {
@@ skipping 75 matching lines @@
   int get length => _base.length;
 
   V putIfAbsent(K key, V ifAbsent()) => _base.putIfAbsent(key, ifAbsent);
 
   V remove(Object key) => _base.remove(key);
 
   Iterable<V> get values => _base.values;
 
   String toString() => _base.toString();
 }