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

pkg/third_party/html5lib/lib/src/css_class_set.dart

 // Copyright (c) 2014, 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.
 
 // TODO(jmesserly): everything in this file is copied straight from "dart:html".
 library html5lib.dom.src;
 
 import 'dart:collection';
 import 'package:html5lib/dom.dart';
 
@@ skipping 261 matching lines @@
 
   Set<String> intersection(Set<String> other) =>
     readClasses().intersection(other);
 
   Set<String> union(Set<String> other) =>
     readClasses().union(other);
 
   Set<String> difference(Set<String> other) =>
     readClasses().difference(other);
 
+  Set<E> cloneEmpty() => new LinkedHashSet<String>();

[Lasse Reichstein Nielsen, 2014/05/19 07:30:05]

@jmesserly:
What does it mean that this is in pkg/third_party?
I expected pkg to be the packages we are maintaining as part of the repository,
so what does third_party mean here? Is this maintained in another repository
somewhere?
(I.e., should I revert the change or should I update the pubspec version?)

[Jennifer Messerly, 2014/05/19 17:44:42]

it means it's under a different license (MIT) and has different authors (listed
in pkg/third_party/html5lib/LICENSE).

IIRC, third_party was advice from the lawyers way back. we tried
"pkg/third_party" vs "third_party/pkg" but it's still confusing. kevmoo was
going to follow up and see if we can do something different. Sorry about the
confusion ... confuses me all the time too :-)

Not sure if we were tracking it, so I opened a bug for Kevin:
https://code.google.com/p/dart/issues/detail?id=18883

+
   String get first => readClasses().first;
   String get last => readClasses().last;
   String get single => readClasses().single;
   List<String> toList({ bool growable: true }) =>
       readClasses().toList(growable: growable);
   Set<String> toSet() => readClasses().toSet();
   Iterable<String> take(int n) => readClasses().take(n);
   Iterable<String> takeWhile(bool test(String value)) =>
       readClasses().takeWhile(test);
   Iterable<String> skip(int n) => readClasses().skip(n);
@@ skipping 35 matching lines @@
    */
   Set<String> readClasses();
 
   /**
    * Join all the elements of a set into one string and write
    * back to the element.
    * This is intended to be overridden by specific implementations.
    */
   void writeClasses(Set<String> s);
 }