Change Set.toSet to always return a set with the same behavior.

pkg/collection/test/wrapper_test.dart

 // Copyright (c) 2013, 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.
 
 /// Tests wrapper utilities.
 
 import "dart:collection";
 import "package:collection/collection.dart";
 import "package:unittest/unittest.dart";
 
@@ skipping 205 matching lines @@
     expect.sublist(4, null).equals.sublist(4);
     expect.sublist(4, 5).equals.sublist(4, 5);
   }
 
   void testSet(var expect) {
     testIterable(expect);
     Set set = new Set();
     expect.add(val).equals.add(val);
     expect.addAll([val]).equals.addAll([val]);
     expect.clear().equals.clear();
+    expect.cloneEmpty().equals.cloneEmpty();
     expect.containsAll([val]).equals.containsAll([val]);
     expect.difference(set).equals.difference(set);
     expect.intersection(set).equals.intersection(set);
     expect.remove(val).equals.remove(val);
     expect.removeAll([val]).equals.removeAll([val]);
     expect.removeWhere(func1).equals.removeWhere(func1);
     expect.retainAll([val]).equals.retainAll([val]);
     expect.retainWhere(func1).equals.retainWhere(func1);
     expect.union(set).equals.union(set);
   }
@@ skipping 42 matching lines @@
   });
 
   test("Queue", () {
     testQueue(new QueueExpector());
   });
 
   test("Map", () {
     testMap(new MapExpector());
   });
 }