Set.add returns true if item has been added, otherwise false

runtime/lib/collection_patch.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.
 
 patch class HashMap<K, V> {
   /* patch */ factory HashMap({ bool equals(K key1, K key2),
                                 int hashCode(K key),
                                 bool isValidKey(potentialKey) }) {
     if (isValidKey == null) {
       if (hashCode == null) {
@@ skipping 553 matching lines @@
     HashSetEntry entry = _buckets[index];
     while (entry != null) {
       if (_equals(entry.key, object)) return true;
       entry = entry.next;
     }
     return false;
   }
 
   // Set.
 
-  void add(E element) {
+  bool add(E element) {
     int hashCode = _hashCode(element);
     int index = hashCode & (_buckets.length - 1);
     HashSetEntry entry = _buckets[index];
     while (entry != null) {
-      if (_equals(entry.key, element)) return;
+      if (_equals(entry.key, element)) return false;
       entry = entry.next;
     }
     _addEntry(element, hashCode, index);
+    return true;
   }
 
   void addAll(Iterable<E> objects) {
     int ctr = 0;
     for (E object in objects) {
       ctr++;
       add(object);
     }
   }
 
@@ skipping 635 matching lines @@
       return false;
     }
     _LinkedHashSetEntry entry = _next;
     _current = entry.key;
     _next = entry._nextEntry;
     return true;
   }
 
   E get current => _current;
 }