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

sdk/lib/core/set.dart

 // Copyright (c) 2011, 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 dart.core;
 
 /**
  * A collection of objects in which each object can occur only once.
  *
  * That is, for each object of the element type, the object is either considered
@@ skipping 57 matching lines @@
   void addAll(Iterable<E> elements);
 
   /**
    * Removes [value] from the set. Returns true if [value] was
    * in the set. Returns false otherwise. The method has no effect
    * if [value] value was not in the set.
    */
   bool remove(Object value);
 
   /**
+   * If an object equal to [object] is in the set, return it.
+   *
+   * Checks if there is an object in the set that is equal to [object].
+   * If so, that object is returned, otherwise returns null.
+   */
+  E lookup(Object object);
+
+  /**
    * Removes each element of [elements] from this set.
    */
   void removeAll(Iterable<Object> elements);
 
   /**
    * Removes all elements of this set that are not elements in [elements].
    */
   void retainAll(Iterable<Object> elements);
 
   /**
@@ skipping 33 matching lines @@
    * That is, the returned set contains all the elements of this `Set` that
    * are not elements of [other].
    */
   Set<E> difference(Set<E> other);
 
   /**
    * Removes all elements in the set.
    */
   void clear();
 }