Make LinkedHashMap also have a factory constructor and be customizable

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 ff6f045fa10404f0234fa17437bee3a69d5b2afe..34744d023591963b2b982bdac1db269c07735a9f 100644
--- a/sdk/lib/collection/linked_hash_map.dart
+++ b/sdk/lib/collection/linked_hash_map.dart
@@ -18,8 +18,10 @@ part of dart.collection;
  *
  * The map allows `null` as a key.
  */
-class LinkedHashMap<K, V> implements HashMap<K, V> {
-  external LinkedHashMap();
+abstract class LinkedHashMap<K, V> implements HashMap<K, V> {
+  external factory LinkedHashMap({ bool equals(K key1, K key2),
+                                   int hashCode(K key),
+                                   bool isValidKey(potentialKey) });
 
   /**
    * Creates a [LinkedHashMap] that contains all key value pairs of [other].
@@ -64,35 +66,4 @@ class LinkedHashMap<K, V> implements HashMap<K, V> {
     Maps._fillMapWithIterables(map, keys, values);
     return map;
   }
-
-  external bool containsKey(Object key);
-
-  external bool containsValue(Object value);
-
-  external void addAll(Map<K, V> other);
-
-  external V operator [](Object key);
-
-  external void operator []=(K key, V value);
-
-  external V putIfAbsent(K key, V ifAbsent());
-
-  external V remove(Object key);
-
-  external void clear();
-
-  external void forEach(void action (K key, V value));
-
-  /** The keys of the map, in insertion order. */
-  external Iterable<K> get keys;
-  /** The values of the map, in the order of their corresponding [keys].*/
-  external Iterable<V> get values;
-
-  external int get length;
-
-  external bool get isEmpty;
-
-  external bool get isNotEmpty;
-
-  String toString() => Maps.mapToString(this);
 }