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

sdk/lib/collection/hash_map.dart

Index: sdk/lib/collection/hash_map.dart
diff --git a/sdk/lib/collection/hash_map.dart b/sdk/lib/collection/hash_map.dart
index bcca966bba5504920f033b4be915596118f83024..775904639b62906d198174ac1d22d3f3b00c267e 100644
--- a/sdk/lib/collection/hash_map.dart
+++ b/sdk/lib/collection/hash_map.dart
@@ -63,6 +63,15 @@ abstract class HashMap<K, V> implements Map<K, V> {
                             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) {