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

sdk/lib/collection/linked_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;
 
 /**
  * A [LinkedHashSet] is a hash-table based [Set] implementation.
  *
  * The `LinkedHashSet` also keep track of the order that elements were inserted
  * in, and iteration happens in first-to-last insertion order.
  *
  * The elements of a `LinkedHashSet` must have consistent [Object.operator==]
  * and [Object.hashCode] implementations. This means that the `==` operator
  * must define a stable equivalence relation on the elements (reflexive,
  * anti-symmetric, transitive, and consistent over time), and that `hashCode`
  * must be the same for objects that are considered equal by `==`.
  *
  * The set allows `null` as an element.
  *
  * Most simple operations on `HashSet` are done in constant time: [add],
  * [contains], [remove], and [length].
  */
 class LinkedHashSet<E> implements HashSet<E> {
 
   external factory LinkedHashSet({ bool equals(E e1, E e2),
                                    int hashCode(E e),
                                    bool isValidKey(potentialKey) });
 
+  /**
+   * Creates an insertion-ordered identity-based set.
+   *
+   * Effectively a shorthand for:
+   *
+   *     new LinkedHashSet(equals: identical, hashCode: identityHashCodeOf)
+   */
+  external factory LinkedHashSet.identity();
+
+
   factory LinkedHashSet.from(Iterable<E> iterable) {
     return new LinkedHashSet<E>()..addAll(iterable);
   }
 }