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

sdk/lib/collection/linked_hash_map.dart

Index: sdk/lib/collection/linked_hash_map.dart
diff --git a/sdk/lib/collection/linked_hash_map.dart b/sdk/lib/collection/linked_hash_map.dart
index 34744d023591963b2b982bdac1db269c07735a9f..ef63bcc3720ceb2b4c041c2438be44ff8fbaf50f 100644
--- a/sdk/lib/collection/linked_hash_map.dart
+++ b/sdk/lib/collection/linked_hash_map.dart
@@ -24,6 +24,14 @@ abstract class LinkedHashMap<K, V> implements HashMap<K, V> {
                                    bool isValidKey(potentialKey) });
 
   /**
+   * Creates an insertion-ordered identity-based map.
+   *
+   * Effectively a shorthand for:
+   *     new LinkedHashMap(equals: identical, hashCode: identityHashCodeOf)
+   */
+  external factory LinkedHashMap.identity();
+
+  /**
    * Creates a [LinkedHashMap] that contains all key value pairs of [other].
    */
   factory LinkedHashMap.from(Map<K, V> other) {