Add Iterable.fold (and Stream.fold) which replace `reduce`.

sdk/lib/html/html_common/filtered_element_list.dart

Index: sdk/lib/html/html_common/filtered_element_list.dart
diff --git a/sdk/lib/html/html_common/filtered_element_list.dart b/sdk/lib/html/html_common/filtered_element_list.dart
index 357ff5faa2f38a6a72eb0e9adab512fd9e336343..43d004deaa98f71e9a55317741eee49a187bf12e 100644
--- a/sdk/lib/html/html_common/filtered_element_list.dart
+++ b/sdk/lib/html/html_common/filtered_element_list.dart
@@ -145,6 +145,12 @@ class FilteredElementList implements List {
       dynamic combine(dynamic previousValue, Element element)) {
     return IterableMixinWorkaround.reduce(this, initialValue, combine);
   }
+
+  dynamic fold(dynamic initialValue,
+      dynamic combine(dynamic previousValue, Element element)) {
+    return IterableMixinWorkaround.fold(this, initialValue, combine);
+  }
+
   bool every(bool f(Element element)) => _filtered.every(f);
   bool any(bool f(Element element)) => _filtered.any(f);
   List<Element> toList({ bool growable: true }) =>