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);

[Lasse Reichstein Nielsen, 2013/04/04 08:35:18]

call fold or readClasses().fold here.

[floitsch, 2013/04/05 16:10:03]

Since we are in a 'reduce' I prefer to call to reduce. When the semantics of
reduce change, I only need to change the number of arguments.

   }
+
+  dynamic fold(dynamic initialValue,
+      dynamic combine(dynamic previousValue, String element)) {
+    return readClasses().fold(initialValue, combine);
+  }
   // interface Collection - END
 
   // interface Set - BEGIN