Reinstall previous behavior for Set and Queue toString.

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 775 matching lines @@
   _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);
   }
 
   void clear() { _clear(); }
 
   Set<E> toSet() => _clone();
 
-  String toString() => _setToString(this);
+  String toString() => IterableBase.iterableToFullString(this, '{', '}');
 }