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

tests/corelib/set_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 set_test;
 
 
 import 'package:expect/expect.dart';
 import "dart:collection";
 
 void testMain(Set create()) {
   testInts(create);
   testStrings(create);
 }
 
 void testInts(Set create()) {
   Set set = create();
 
   testLength(0, set);
-  set.add(1);
+  Expect.isTrue(set.add(1));
   testLength(1, set);
   Expect.isTrue(set.contains(1));
 
-  set.add(1);
+  Expect.isFalse(set.add(1));
   testLength(1, set);
   Expect.isTrue(set.contains(1));
 
-  set.remove(1);
+  Expect.isTrue(set.remove(1));
   testLength(0, set);
   Expect.isFalse(set.contains(1));
 
+  Expect.isFalse(set.remove(1));
+  testLength(0, set);
+  Expect.isFalse(set.contains(1));
+
   for (int i = 0; i < 10; i++) {
     set.add(i);
   }
 
   testLength(10, set);
   for (int i = 0; i < 10; i++) {
     Expect.isTrue(set.contains(i));
   }
 
   testLength(10, set);
@@ skipping 135 matching lines @@
   for (int i = 0; i < 10; i++) {
     Expect.isTrue(set.contains(i));
   }
   for (int i = 10; i < 20; i++) {
     Expect.isFalse(set.contains(i));
   }
 
   // Test Set.clear.
   set.clear();
   testLength(0, set);
-  set.add(11);
+  Expect.isTrue(set.add(11));
   testLength(1, set);
 }
 
 void testLength(int length, Set set) {
   Expect.equals(length, set.length);
   (length == 0 ? Expect.isTrue : Expect.isFalse)(set.isEmpty);
   (length != 0 ? Expect.isTrue : Expect.isFalse)(set.isNotEmpty);
   if (length == 0) {
     for (var e in set) { Expect.fail("contains element when iterated: $e"); }
   }
   (length == 0 ? Expect.isFalse : Expect.isTrue)(set.iterator.moveNext());
 }
 
 void testStrings(Set create()) {
   var set = create();
   var strings = ["foo", "bar", "baz", "qux", "fisk", "hest", "svin", "pigvar"];
   set.addAll(strings);
   testLength(8, set);
   set.removeAll(strings.where((x) => x.length == 3));
   testLength(4, set);
-  set.add("bar");
-  set.add("qux");
+  Expect.isTrue(set.add("bar"));
+  Expect.isTrue(set.add("qux"));
   testLength(6, set);
   set.addAll(strings);
   testLength(8, set);
   set.removeWhere((x) => x.length != 3);
   testLength(4, set);
   set.retainWhere((x) => x[1] == "a");
   testLength(2, set);
   Expect.isTrue(set.containsAll(["baz", "bar"]));
 
   set = set.union(strings.where((x) => x.length != 3).toSet());
@@ skipping 107 matching lines @@
   testTypeAnnotations(new LinkedHashSet<int>(equals: (int a, int b) => a == b,
                                              hashCode: (int a) => a.hashCode,
                                              isValidKey: (a) => a is int));
 
   testRetainWhere(([equals, hashCode, validKey]) =>
       new HashSet(equals: equals, hashCode: hashCode, isValidKey: validKey));
   testRetainWhere(([equals, hashCode, validKey]) =>
       new LinkedHashSet(equals: equals, hashCode: hashCode,
                         isValidKey: validKey));
 }