Revert "Move toString() to collection classes."

sdk/lib/collection/hash_set.dart

Index: sdk/lib/collection/hash_set.dart
diff --git a/sdk/lib/collection/hash_set.dart b/sdk/lib/collection/hash_set.dart
index 147d1ca07e6e07f98cd07af48572c7364bf8eb4b..86c69c81ee5f4346fea08f0b42fbfc8bec58300f 100644
--- a/sdk/lib/collection/hash_set.dart
+++ b/sdk/lib/collection/hash_set.dart
@@ -6,7 +6,6 @@ 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) {
@@ -54,8 +53,7 @@ abstract class _HashSetBase<E> extends IterableBase<E> implements Set<E> {
     retainWhere(retainSet.contains);
   }
 
-  // TODO(zarah) Remove this, and let it be inherited by IterableBase
-  String toString() => IterableMixinWorkaround.toStringIterable(this, '{', '}');
+  String toString() => ToString.iterableToString(this);
 }
 
 /**