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

pkg/unmodifiable_collection/lib/unmodifiable_collection.dart

 // Copyright (c) 2013, 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.
 
 /**
  * Wrappers that prevent List, Set, or Map objects from being modified.
  *
  * The [Set] and [Map] wrappers allow reading from the wrapped collection,
  * but prohibit writing.
  *
@@ skipping 161 matching lines @@
   }
 
   bool containsAll(Iterable<E> other) => _source.containsAll(other);
 
   Set<E> intersection(Set<E> other) => _source.intersection(other);
 
   Set<E> union(Set<E> other) => _source.union(other);
 
   Set<E> difference(Set<E> other) => _source.difference(other);
 
+  E lookup(Object object) => _source.lookup(object);
 
   /**
    * Throws an [UnsupportedError];
    * operations that change the set are disallowed.
    */
   void add(E value) => _throw();
 
   /**
    * Throws an [UnsupportedError];
    * operations that change the set are disallowed.
@@ skipping 158 matching lines @@
   Iterable<E> take(int n) => _source.take(n);
 
   Iterable<E> takeWhile(bool test(E value)) => _source.takeWhile(test);
 
   List<E> toList({ bool growable: true }) => _source.toList(growable: growable);
 
   Set<E> toSet() => _source.toSet();
 
   Iterable<E> where(bool test(E element)) => _source.where(test);
 }