Make EfficientLength extend Iterable.

sdk/lib/_internal/js_runtime/lib/collection_patch.dart

Index: sdk/lib/_internal/js_runtime/lib/collection_patch.dart
diff --git a/sdk/lib/_internal/js_runtime/lib/collection_patch.dart b/sdk/lib/_internal/js_runtime/lib/collection_patch.dart
index beb67f242d1d6437db5aee3cda1d6bdb95ef2725..0d61257018465e63b6d8e2dfbe38e57e146537af 100644
--- a/sdk/lib/_internal/js_runtime/lib/collection_patch.dart
+++ b/sdk/lib/_internal/js_runtime/lib/collection_patch.dart
@@ -431,8 +431,7 @@ class _CustomHashMap<K, V> extends _HashMap<K, V> {
   String toString() => Maps.mapToString(this);
 }
 
-class _HashMapKeyIterable<E> extends Iterable<E>
-                             implements EfficientLength {
+class _HashMapKeyIterable<E> extends EfficientLengthIterable<E> {
   final _map;
   _HashMapKeyIterable(this._map);
 
@@ -664,8 +663,7 @@ class _Es6LinkedIdentityHashMap<K, V>
   String toString() => Maps.mapToString(this);
 }
 
-class _Es6MapIterable<E> extends Iterable<E>
-                         implements EfficientLength {
+class _Es6MapIterable<E> extends EfficientLengthIterable<E> {
   final _map;
   final bool _isKeys;