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

sdk/lib/collection/linked_hash_map.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 hash-table based implementation of [Map].
  *
  * Keys insertion order is remembered, and keys are iterated in insertion order.
  * Values are iterated in their corresponding key's order.
  *
  * The keys of a `LinkedHashMap` must have consistent [Object.operator==]
  * and [Object.hashCode] implementations. This means that the `==` operator
  * must define a stable equivalence relation on the keys (reflexive,
  * anti-symmetric, transitive, and consistent over time), and that `hashCode`
  * must be the same for objects that are considered equal by `==`.
  *
  * The map allows `null` as a key.
  */
 abstract class LinkedHashMap<K, V> implements HashMap<K, V> {
   external factory LinkedHashMap({ bool equals(K key1, K key2),
                                    int hashCode(K key),
                                    bool isValidKey(potentialKey) });
 
   /**
+   * Creates an insertion-ordered identity-based map.
+   *
+   * Effectively a shorthand for:
+   *     new LinkedHashMap(equals: identical, hashCode: identityHashCodeOf)
+   */
+  external factory LinkedHashMap.identity();
+
+  /**
    * Creates a [LinkedHashMap] that contains all key value pairs of [other].
    */
   factory LinkedHashMap.from(Map<K, V> other) {
     return new LinkedHashMap<K, V>()..addAll(other);
   }
 
   /**
    * Creates a [LinkedHashMap] where the keys and values are computed from the
    * [iterable].
    *
@@ skipping 23 matching lines @@
    * overwrites the previous value.
    *
    * It is an error if the two [Iterable]s don't have the same length.
    */
   factory LinkedHashMap.fromIterables(Iterable<K> keys, Iterable<V> values) {
     LinkedHashMap<K, V> map = new LinkedHashMap<K, V>();
     Maps._fillMapWithIterables(map, keys, values);
     return map;
   }
 }