Change Set.add to return a boolean indicating success.

corelib/src/implementation/hash_map_set.dart

 // Copyright (c) 2012, 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.
 
 // Hash map implementation with open addressing and quadratic probing.
 class HashMapImplementation<K extends Hashable, V> implements HashMap<K, V> {
 
   // The [_keys] list contains the keys inserted in the map.
   // The [_keys] list must be a raw list because it
   // will contain both elements of type K, and the [_DELETED_KEY] of type
@@ skipping 274 matching lines @@
     for (final e in other) {
       set.add(e);
     }
     return set;
   }
 
   void clear() {
     _backingMap.clear();
   }
 
-  void add(E value) {
+  bool add(E value) {
+    if (_backingMap.containsKey(value)) return false;
     _backingMap[value] = value;
+    return true;
   }
 
   bool contains(E value) {
     return _backingMap.containsKey(value);
   }
 
   bool remove(E value) {
     if (!_backingMap.containsKey(value)) return false;
     _backingMap.remove(value);
     return true;
@@ skipping 133 matching lines @@
 
 /**
  * A singleton sentinel used to represent when a key is deleted from the map.
  * We can't use [: const Object() :] as a sentinel because it would end up
  * canonicalized and then we cannot distinguish the deleted key from the
  * canonicalized [: Object() :].
  */
 class _DeletedKeySentinel {
   const _DeletedKeySentinel();
 }