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

sdk/lib/html/html_common/css_class_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.
 
 part of html_common;
 
 abstract class CssClassSetImpl implements CssClassSet {
 
   String toString() {
     return readClasses().join(' ');
@@ skipping 154 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);
 
+  // Do overwrite this method in subclasses if it is not the type of set
+  // returned by readClasses.
+  Set<String> cloneEmpty() => new LinkedHashSet<String>();
+
   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);
 }