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

tools/dom/templates/immutable_list_mixin.darttemplate

Index: tools/dom/templates/immutable_list_mixin.darttemplate
diff --git a/tools/dom/templates/immutable_list_mixin.darttemplate b/tools/dom/templates/immutable_list_mixin.darttemplate
index d238fcaab12a1bd45c7305c30dd880607f209b4b..81c72e257f465018c0f7f6ead1762dee63d17257 100644
--- a/tools/dom/templates/immutable_list_mixin.darttemplate
+++ b/tools/dom/templates/immutable_list_mixin.darttemplate
@@ -18,6 +18,10 @@ $endif
     return IterableMixinWorkaround.reduce(this, initialValue, combine);
   }
 
+  dynamic fold(dynamic initialValue, dynamic combine(dynamic, $E)) {
+    return IterableMixinWorkaround.fold(this, initialValue, combine);
+  }
+
 $if DEFINE_CONTAINS
   bool contains($E element) => IterableMixinWorkaround.contains(this, element);
 $else