Update implementations to use Object.identityHashCode for identity map/set

sdk/lib/collection/hash_set.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.
 
 part of dart.collection;
 
 /** Common parts of [HashSet] and [LinkedHashSet] implementations. */
 abstract class _HashSetBase<E> extends IterableBase<E> implements Set<E> {
 
   // Set.
@@ skipping 92 matching lines @@
    *
    * If [equals] is [identical], this creates an identity set. Any hashCode
    * is compatible with [identical], and it applies to all objects, so
    * [hashCode] and [isValidKey] can safely be omitted.
    */
   external factory HashSet({ bool equals(E e1, E e2),
                              int hashCode(E e),
                              bool isValidKey(potentialKey) });
 
   /**
+   * Creates an unordered identity-based set.
+   *
+   * Effectively a shorthand for:
+   *
+   *     new HashSet(equals: identical, hashCode: identityHashCodeOf)
+   */
+  external factory HashSet.identity();
+
+  /**
    * Create a hash set containing the elements of [iterable].
    *
    * Creates a hash set as by `new HashSet<E>()` and adds each element of
    * `iterable` to this set in the order they are iterated.
    */
   factory HashSet.from(Iterable<E> iterable) {
     return new HashSet<E>()..addAll(iterable);
   }
 }