Revert "Make EfficientLength public, as EfficientLengthIterable."

sdk/lib/_internal/compiler/js_lib/collection_patch.dart

Index: sdk/lib/_internal/compiler/js_lib/collection_patch.dart
diff --git a/sdk/lib/_internal/compiler/js_lib/collection_patch.dart b/sdk/lib/_internal/compiler/js_lib/collection_patch.dart
index 06d944a36e4cebb20efe4f277df242a3e381016c..9df7c993393011693306552cfbee61454f19a616 100644
--- a/sdk/lib/_internal/compiler/js_lib/collection_patch.dart
+++ b/sdk/lib/_internal/compiler/js_lib/collection_patch.dart
@@ -430,7 +430,7 @@ class _CustomHashMap<K, V> extends _HashMap<K, V> {
 }
 
 class HashMapKeyIterable<E> extends Iterable<E>
-                            implements EfficientLengthIterable<E> {
+                            implements EfficientLength {
   final _map;
   HashMapKeyIterable(this._map);