Change Set.add to return a boolean indicating success.

corelib/src/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.
  */
 interface Set<E> extends Collection<E>
     default HashSetImplementation<E extends Hashable> {
   Set();
 
   /**
    * Creates a [Set] that contains all elements of [other].
    */
   Set.from(Iterable<E> other);
 
   /**
    * Returns true if [value] is in the set.
    */
   bool contains(E value);
 
   /**
-   * Adds [value] into the set. The method has no effect if
-   * [value] was already in the set.
+   * Adds [value] into the set. Returns true if [value] was not in the set.
+   * The method has no effect if [value] was already in the set.
    */
-  void add(E value);
+  bool add(E value);
 
   /**
    * Removes [value] from the set. Returns true if [value] was
    * in the set. Returns false otherwise. The method has no effect
    * if [value] value was not in the set.
    */
   bool remove(E value);
 
   /**
    * Adds all the elements of the given collection to the set.
@@ skipping 32 matching lines @@
 
 interface HashSet<E extends Hashable> extends Set<E>
     default HashSetImplementation<E extends Hashable> {
   HashSet();
 
   /**
    * Creates a [Set] that contains all elements of [other].
    */
   HashSet.from(Iterable<E> other);
 }