Remove uses of IterableMixinWorkaround outside of VM and dart2js lists.

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 SetBase<E> {
 
   // The following two methods override the ones in SetBase.
@@ skipping 14 matching lines @@
       if (other.contains(element)) result.add(element);
     }
     return result;
   }
 
   Set<E> _newSet();
 
   // Subclasses can optimize this further.
   Set<E> toSet() => _newSet()..addAll(this);
 
-  String toString() => IterableMixinWorkaround.toStringIterable(this, '{', '}');
+  String toString() => _setToString(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
  * must define a stable equivalence relation on the elements (reflexive,
  * anti-symmetric, transitive, and consistent over time), and that the hashCode
  * must consistent with equality, so that the same for objects that are
@@ skipping 51 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;
 }