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

sdk/lib/collection/linked_hash_set.dart

Index: sdk/lib/collection/linked_hash_set.dart
diff --git a/sdk/lib/collection/linked_hash_set.dart b/sdk/lib/collection/linked_hash_set.dart
index d9aadf2c3e54cf0993cc3d5be433b7883acb42e4..7a79991171cc80bc54a80da1590d630c22fede20 100644
--- a/sdk/lib/collection/linked_hash_set.dart
+++ b/sdk/lib/collection/linked_hash_set.dart
@@ -27,6 +27,16 @@ class LinkedHashSet<E> implements HashSet<E> {
                                    int hashCode(E e),
                                    bool isValidKey(potentialKey) });
 
+  /**
+   * Creates an insertion-ordered identity-based set.
+   *
+   * Effectively a shorthand for:
+   *
+   *     new LinkedHashSet(equals: identical, hashCode: identityHashCodeOf)
+   */
+  external factory LinkedHashSet.identity();
+
+
   factory LinkedHashSet.from(Iterable<E> iterable) {
     return new LinkedHashSet<E>()..addAll(iterable);
   }