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

tests/html/element_classes_test.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.
 
 library ElementTest;
 import '../../pkg/unittest/lib/unittest.dart';
 import '../../pkg/unittest/lib/html_config.dart';
 import 'dart:collection';
 import 'dart:html';
 
@@ skipping 89 matching lines @@
     expect(makeElement().classes.length, 0);
   });
 
   test('contains', () {
     expect(makeClassSet().contains('foo'), isTrue);
     expect(makeClassSet().contains('qux'), isFalse);
   });
 
   test('add', () {
     final classes = makeClassSet();
-    classes.add('qux');
+    var added = classes.add('qux');
+    expect(added, isTrue);
     expect(classes, orderedEquals(['foo', 'bar', 'baz', 'qux']));
 
-    classes.add('qux');
+    added = classes.add('qux');
+    expect(added, isFalse);
     final list = new List.from(classes);
     list.sort((a, b) => a.compareTo(b));
     expect(list, orderedEquals(['bar', 'baz', 'foo', 'qux']),
         reason: "The class set shouldn't have duplicate elements.");
   });
 
   test('remove', () {
     final classes = makeClassSet();
     classes.remove('bar');
     expect(classes, orderedEquals(['foo', 'baz']));
@@ skipping 104 matching lines @@
   test('listContains', () {
     var elements = listElementSetup();
     expect(elements.classes.contains('lassy'), isTrue);
     expect(elements.classes.contains('foo'), isFalse);
     document.documentElement.children.remove(listElement);
   });
 
 
   test('listAdd', () {
     var elements =  listElementSetup();
-    elements.classes.add('lassie');
+    var added = elements.classes.add('lassie');
+    expect(added, isNull);
+
     expect(elements.classes,
         unorderedEquals(['lassie', 'qux', 'quux', 'meta', 'classy', 'lassy']));
     for (Element e in elements) {
       expect(e.classes, anyOf(unorderedEquals(['quux', 'qux', 'lassie']),
           unorderedEquals(['meta', 'lassie']),
           unorderedEquals(['classy', 'lassy', 'lassie']),
           unorderedEquals(['qux', 'lassy', 'lassie'])));
     }
     document.documentElement.children.remove(listElement);
   });
@@ skipping 75 matching lines @@
     document.documentElement.children.remove(listElement);
   });
 
   test('listContainsAll', () {
     var elements = listElementSetup();
     expect(elements.classes.containsAll(['qux', 'meta', 'mornin']), isFalse);
     expect(elements.classes.containsAll(['qux', 'lassy', 'classy']), isTrue);
     document.documentElement.children.remove(listElement);
   });
 }