Fix identical test in hash-set.

sdk/lib/core/set.dart

 // Copyright (c) 2011, 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.
 
 /**
  * This class is the public interface of a set. A set is a collection
  * without duplicates.
  */
 abstract class Set<E> extends Collection<E> {
   factory Set() => new _HashSetImpl<E>();
@@ skipping 191 matching lines @@
 
   // TODO(4504458): Replace set_ with set.
   _HashSetIterator(_HashSetImpl<E> set_)
     : _nextValidIndex = -1,
       _entries = set_._backingMap._keys {
     _advance();
   }
 
   bool get hasNext {
     if (_nextValidIndex >= _entries.length) return false;
-    if (_entries[_nextValidIndex] == _HashMapImpl._DELETED_KEY) {
+    if (identical(_entries[_nextValidIndex], _HashMapImpl._DELETED_KEY)) {
       // This happens in case the set was modified in the meantime.
       // A modification on the set may make this iterator misbehave,
       // but we should never return the sentinel.
       _advance();
     }
     return _nextValidIndex < _entries.length;
   }
 
   E next() {
     if (!hasNext) {
@@ skipping 15 matching lines @@
   }
 
   // The entries in the set. May contain null or the sentinel value.
   List<E> _entries;
 
   // The next valid index in [_entries] or the length of [entries_].
   // If it is the length of [_entries], calling [hasNext] on the
   // iterator will return false.
   int _nextValidIndex;
 }