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

sdk/lib/collection/hash_set.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.
 
 part of dart.collection;
 
 /** Common parts of [HashSet] and [LinkedHashSet] implementations. */
 abstract class _HashSetBase<E> extends IterableBase<E> implements Set<E> {
 
   // Set.
@@ skipping 40 matching lines @@
   }
 
   List<E> toList({bool growable: true}) {
     List<E> result = growable ? (new List<E>()..length = this.length)
                               : new List<E>(this.length);
     int i = 0;
     for (E element in this) result[i++] = element;
     return result;
   }
 
+  Set<E> cloneEmpty() => _newSet();
+
   Set<E> toSet() => _newSet()..addAll(this);
 
   String toString() => IterableMixinWorkaround.toStringIterable(this, '{', '}');
 }
 
 /**
  * An unordered hash-table based [Set] implementation.
  *
  * The elements of a `HashSet` must have consistent equality
  * and hashCode implementations. This means that the equals operation
@@ skipping 54 matching lines @@
   }
 
   /**
    * Provides an iterator that iterates over the elements of this set.
    *
    * The order of iteration is unspecified,
    * but consistent between changes to the set.
    */
   Iterator<E> get iterator;
 }