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

tools/dom/src/CssClassSet.dart

Index: tools/dom/src/CssClassSet.dart
diff --git a/tools/dom/src/CssClassSet.dart b/tools/dom/src/CssClassSet.dart
index 473c0370d1036b3ca20180d8d289e64fdf191dc3..29aa8602dd2cededcb19b7c9811b8aba4dd60994 100644
--- a/tools/dom/src/CssClassSet.dart
+++ b/tools/dom/src/CssClassSet.dart
@@ -62,6 +62,11 @@ abstract class CssClassSet implements Set<String> {
       dynamic combine(dynamic previousValue, String element)) {
     return readClasses().reduce(initialValue, combine);
   }
+
+  dynamic fold(dynamic initialValue,
+      dynamic combine(dynamic previousValue, String element)) {
+    return readClasses().fold(initialValue, combine);
+  }
   // interface Collection - END
 
   // interface Set - BEGIN