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

sdk/lib/core/set.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.core;
 
 /**
  * A collection of objects in which each object can occur only once.
  *
  * That is, for each object of the element type, the object is either considered
@@ skipping 148 matching lines @@
    *
    * That is, the returned set contains all the elements of this [Set] that
    * are not elements of [other] according to `other.contains`.
    */
   Set<E> difference(Set<E> other);
 
   /**
    * Removes all elements in the set.
    */
   void clear();
+
+  /**
+   * Create a set of the same type as this one, but with no elements.
+   *
+   * The new set has the same behavior as this set,
+   * but starts out with no elements.
+   *
+   * This method should be equivalent to `toSet()..clear()`.
+   */
+  Set<E> cloneEmpty();
+
+  /* Creates a [Set] with the same elements and behavior as this `Set`.
+   *
+   * The returned set behaves the same as this set
+   * with regard to adding and removing elements.
+   * It initially contains the same elements.
+   * If this set specifies an ordering of the elements,
+   * the returned set will have the same order.
+   */
+  Set<E> toSet();
 }