Fix CssClassSet methods to accept Set, not Collection.

tools/dom/src/CssClassSet.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;
 
 abstract class CssClassSet implements Set<String> {
 
   String toString() {
     return readClasses().join(' ');
@@ skipping 90 matching lines @@
   void retainWhere(bool test(String name)) {
     _modify((s) => s.retainWhere(test));
   }
 
   bool isSubsetOf(Collection<String> collection) =>
     readClasses().isSubsetOf(collection);
 
   bool containsAll(Iterable<String> collection) =>
     readClasses().containsAll(collection);
 
-  Set<String> intersection(Collection<String> other) =>
+  Set<String> intersection(Set<String> other) =>
     readClasses().intersection(other);
 
-  Set<String> union(Collection<String> other) =>
+  Set<String> union(Set<String> other) =>
     readClasses().union(other);
 
-  Set<String> difference(Collection<String> other) =>
+  Set<String> difference(Set<String> other) =>
     readClasses().difference(other);
 
   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();
   String min([int compare(String a, String b)]) =>
       readClasses().min(compare);
@@ skipping 40 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);
 }