Add Set.lookup method to get the instance in the set.

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 65 matching lines @@
 
   // interface Set - BEGIN
   /**
    * Determine if this element contains the class [value].
    *
    * This is the Dart equivalent of jQuery's
    * [hasClass](http://api.jquery.com/hasClass/).
    */
   bool contains(String value) => readClasses().contains(value);
 
+  /** Lookup from the Set interface. Not interesting for a String set. */
+  E lookup(String value) => contains(value) ? value : null;
+
   /**
    * Add the class [value] to element.
    *
    * This is the Dart equivalent of jQuery's
    * [addClass](http://api.jquery.com/addClass/).
    */
   void add(String value) {
     // TODO - figure out if we need to do any validation here
     // or if the browser natively does enough.
     modify((s) => s.add(value));
@@ skipping 120 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);
 }