Make EfficientLength extend Iterable.

sdk/lib/_internal/js_runtime/lib/linked_hash_map.dart

Index: sdk/lib/_internal/js_runtime/lib/linked_hash_map.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/linked_hash_map.dart b/sdk/lib/_internal/js_runtime/lib/linked_hash_map.dart
index ada768fdacc060a78ec033958dde28e9bb5ac7d6..98fad0529fcd0a313b2226fd2c87d0c841eaf968 100644
--- a/sdk/lib/_internal/js_runtime/lib/linked_hash_map.dart
+++ b/sdk/lib/_internal/js_runtime/lib/linked_hash_map.dart
@@ -380,8 +380,7 @@ class LinkedHashMapCell<K, V> {
   LinkedHashMapCell(this.hashMapCellKey, this.hashMapCellValue);
 }
 
-class LinkedHashMapKeyIterable<E> extends Iterable<E>
-    implements EfficientLength {
+class LinkedHashMapKeyIterable<E> extends EfficientLengthIterable<E> {
   final dynamic/*=JsLinkedHashMap<E, dynamic>*/ _map;
   LinkedHashMapKeyIterable(this._map);