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

sdk/lib/collection/splay_tree.dart

 // Copyright (c) 2012, 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;
 
 typedef bool _Predicate<T>(T value);
 
 /**
  * A node in a splay tree. It holds the sorting key and the left
@@ skipping 767 matching lines @@
   }
 
   E lookup(Object object) {
     if (!_validKey(object)) return null;
     int comp = _splay(object);
     if (comp != 0) return null;
     return _root.key;
   }
 
   Set<E> intersection(Set<E> other) {
-    Set<E> result = new SplayTreeSet<E>(_compare, _validKey);
+    Set<E> result = new SplayTreeSet<E>(_comparator, _validKey);
     for (E element in this) {
       if (other.contains(element)) result.add(element);
     }
     return result;
   }
 
   Set<E> difference(Set<E> other) {
-    Set<E> result = new SplayTreeSet<E>(_compare, _validKey);
+    Set<E> result = new SplayTreeSet<E>(_comparator, _validKey);
     for (E element in this) {
       if (!other.contains(element)) result.add(element);
     }
     return result;
   }
 
   Set<E> union(Set<E> other) {
     return _clone()..addAll(other);
   }
 
   SplayTreeSet<E> _clone() {
-    var set = new SplayTreeSet<E>(_compare, _validKey);
+    var set = new SplayTreeSet<E>(_comparator, _validKey);
     set._count = _count;
     set._root = _cloneNode(_root);
     return set;
   }
 
   _SplayTreeNode<E> _cloneNode(_SplayTreeNode<E> node) {
     if (node == null) return null;
     return new _SplayTreeNode<E>(node.key)..left = _cloneNode(node.left)
                                           ..right = _cloneNode(node.right);
   }
 
   bool containsAll(Iterable<Object> other) {
     for (var element in other) {
       if (!this.contains(element)) return false;
     }
     return true;
   }
 
   void clear() { _clear(); }
+
+  Set<E> toSet() => _clone();
 }