Reinstall previous behavior for Set and Queue toString.

sdk/lib/collection/set.dart

 // Copyright (c) 2014, 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.
 
 /**
  * Base implementations of [Set].
  */
 part of dart.collection;
 
 /**
@@ skipping 113 matching lines @@
       new EfficientLengthMappedIterable<E, dynamic>(this, f);
 
   E get single {
     if (length > 1) throw IterableElementError.tooMany();
     Iterator it = iterator;
     if (!it.moveNext()) throw IterableElementError.noElement();
     E result = it.current;
     return result;
   }
 
-  String toString() => _setToString(this);
+  String toString() => IterableBase.iterableToFullString(this, '{', '}');
 
   // Copied from IterableMixin.
   // Should be inherited if we had multi-level mixins.
 
   Iterable<E> where(bool f(E element)) => new WhereIterable<E>(this, f);
 
   Iterable expand(Iterable f(E element)) =>
       new ExpandIterable<E, dynamic>(this, f);
 
   void forEach(void f(E element)) {
@@ skipping 146 matching lines @@
  * Some of the methods assume that `toSet` creates a modifiable set.
  * If using this base class for an unmodifiable set,
  * where `toSet` should return an unmodifiable set,
  * it's necessary to reimplement
  * [retainAll], [union], [intersection] and [difference].
  *
  * Implementations of `Set` using this base should consider also implementing
  * `clear` in constant time. The default implementation works by removing every
  * element.
  */
-abstract class SetBase<E> = Object with SetMixin<E>;
+abstract class SetBase<E> extends SetMixin<E> {
+  /**
+   * Convert a `Set` to a string as `{each, element, as, string}`.
+   *
+   * Handles circular references where converting one of the elements
+   * to a string ends up converting [set] to a string again.
+   */
+  static String setToString(Set set) =>
+      IterableBase.iterableToFullString(set, '{', '}');
+}