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

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.
 
 part of dart.core;
 
 /**
  * A collection of objects in which each object can occur only once.
  *
  * That is, for each object of the element type, the object is either considered
@@ skipping 41 matching lines @@
    * have a compatible [Object.hashCode] implementation.
    */
   factory Set.from(Iterable<E> other) = LinkedHashSet<E>.from;
 
   /**
    * Returns true if [value] is in the set.
    */
   bool contains(Object value);
 
   /**
-   * Adds [value] into the set.
+   * Adds [value] into the set. Returns `true` if [value] was added to the set.
    *
-   * The method has no effect if [value] is already in the set.
+   * If [value] already exists, the set is not changed and `false` is returned.
    */
-  void add(E value);
+  bool add(E value);
 
   /**
    * Adds all of [elements] to this Set.
    *
    * Equivalent to adding each element in [elements] using [add],
    * but some collections may be able to optimize it.
    */
   void addAll(Iterable<E> elements);
 
   /**
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    * That is, the returned set contains all the elements of this `Set` that
    * are not elements of [other].
    */
   Set<E> difference(Set<E> other);
 
   /**
    * Removes all elements in the set.
    */
   void clear();
 }