Remove deprecated Strings class.

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 Strings.join(new List.from(readClasses()), ' ');
+    return readClasses().join(' ');
   }
 
   /**
    * Adds the class [token] to the element if it is not on it, removes it if it
    * is.
    */
   bool toggle(String value) {
     Set<String> s = readClasses();
     bool result = false;
     if (s.contains(value)) {
@@ skipping 140 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);
 }