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

sdk/lib/collection/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;
 
 /** Default function for equality comparison in customized HashMaps */
 bool _defaultEquals(a, b) => a == b;
 /** Default function for hash-code computation in customized HashMaps */
 int _defaultHashCode(a) => a.hashCode;
@@ skipping 45 matching lines @@
    *
    * It is generally the case that if you supply one of [equals] and [hashCode],
    * you also want to supply the other. The only common exception is to pass
    * [identical] as the equality and use the default hash code.
    */
   external factory HashMap({bool equals(K key1, K key2),
                             int hashCode(K key),
                             bool isValidKey(potentialKey)});
 
   /**
+   * Creates an unordered identity-based map.
+   *
+   * Effectively a shorthand for:
+   *
+   *     new HashMap(equals: identical, hashCode: identityHashCodeOf)
+   */
+  external factory HashMap.identity();
+
+  /**
    * Creates a [HashMap] that contains all key value pairs of [other].
    */
   factory HashMap.from(Map<K, V> other) {
     return new HashMap<K, V>()..addAll(other);
   }
 
   /**
    * Creates a [HashMap] 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 HashMap.fromIterables(Iterable<K> keys, Iterable<V> values) {
     HashMap<K, V> map = new HashMap<K, V>();
     Maps._fillMapWithIterables(map, keys, values);
     return map;
   }
 }