a === b -> identical(a, b)

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 216 matching lines @@
     return res;
   }
 
   void _advance() {
     int length = _entries.length;
     var entry;
     final deletedKey = _HashMapImpl._DELETED_KEY;
     do {
       if (++_nextValidIndex >= length) break;
       entry = _entries[_nextValidIndex];
-    } while ((entry === null) || (entry === deletedKey));
+    } while ((entry == null) || identical(entry, deletedKey));
   }
 
   // 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;
 }