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

sdk/lib/collection/hash_set.dart

Index: sdk/lib/collection/hash_set.dart
diff --git a/sdk/lib/collection/hash_set.dart b/sdk/lib/collection/hash_set.dart
index f1241a84e45e190b20f9ad2a65b43ae27a380dd0..36d23d2de951ae97603aae1303d4878d75969ff4 100644
--- a/sdk/lib/collection/hash_set.dart
+++ b/sdk/lib/collection/hash_set.dart
@@ -110,6 +110,14 @@ class HashSet<E> implements Set<E> {
                              bool isValidKey(potentialKey) });
 
   /**
+   * Creates an unordered identity-based set.
+   *
+   * Effectively a shorthand for:
+   *     new HashSet(equals: identical, hashCode: identityHashCodeOf)
+   */
+  external factory HashSet.identity();
+
+  /**
    * Create a hash set containing the elements of [iterable].
    *
    * Creates a hash set as by `new HashSet<E>()` and adds each element of