Revert "Move toString() to collection classes."

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.
   bool containsAll(Iterable<Object> other) {
     for (Object object in other) {
       if (!this.contains(object)) return false;
     }
     return true;
   }
 
   /** Create a new Set of the same type as this. */
   Set _newSet();
@@ skipping 27 matching lines @@
   void retainAll(Iterable objectsToRetain) {
     Set retainSet;
     if (objectsToRetain is Set) {
       retainSet = objectsToRetain;
     } else {
       retainSet = objectsToRetain.toSet();
     }
     retainWhere(retainSet.contains);
   }
 
-  // TODO(zarah) Remove this, and let it be inherited by IterableBase
-  String toString() => IterableMixinWorkaround.toStringIterable(this, '{', '}');
+  String toString() => ToString.iterableToString(this);
 }
 
 /**
  * A [HashSet] is a hash-table based [Set] implementation.
  *
  * The elements of a `HashSet` must have consistent [Object.operator==]
  * and [Object.hashCode] implementations. This means that the `==` operator
  * must define a stable equivalence relation on the elements (reflexive,
  * anti-symmetric, transitive, and consistent over time), and that `hashCode`
  * must be the same for objects that are considered equal by `==`.
@@ skipping 31 matching lines @@
   external void removeAll(Iterable<Object> objectsToRemove);
 
   external void removeWhere(bool test(E element));
 
   external void retainWhere(bool test(E element));
 
   external void clear();
 
   Set<E> _newSet() => new HashSet<E>();
 }