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

sdk/lib/core/map.dart

 // Copyright (c) 2011, 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.core;
 
 /**
  * An unordered collection of key-value pairs,
  * from which you retrieve a value by using its associated key.
  *
  * Each key can occur at most once in a map.
  */
 abstract class Map<K, V> {
   /**
    * Creates a Map instance with the default implementation.
    */
   factory Map() = LinkedHashMap<K, V>;
 
   /**
    * Creates a Map instance that contains all key-value pairs of [other].
    */
   factory Map.from(Map<K, V> other) = LinkedHashMap<K, V>.from;
 
   /**
+   * Creates an identity map with the default implementation.
+   */
+  factory Map.identity() = LinkedHashMap<K, V>.identity;
+
+  /**
    * Creates a Map instance
    * where the keys and values are computed from the [iterable].
    *
    * For each element of the [iterable] this constructor computes a key-value
    * pair, by applying [key] and [value] respectively.
    *
    * The keys computed by the source [iterable]
    * do not need to be unique. The last
    * occurrence of a key will simply overwrite any previous value.
    *
@@ skipping 99 matching lines @@
   /**
    * Returns true if there is no {key, value} pair in the map.
    */
   bool get isEmpty;
 
   /**
    * Returns true if there is at least one {key, value} pair in the map.
    */
   bool get isNotEmpty;
 }