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

tests/html/streams_test.dart

Index: tests/html/streams_test.dart
diff --git a/tests/html/streams_test.dart b/tests/html/streams_test.dart
index 266edc8146d35799b98366f3f019ee8edb94f314..3d3f16c46962b70a99daff17c268c8e1f21023fd 100644
--- a/tests/html/streams_test.dart
+++ b/tests/html/streams_test.dart
@@ -194,6 +194,10 @@ main() {
     stream.reduce(null, (a, b) => null).then((_) {});
   });
 
+  test('fold', () {
+    stream.fold(null, (a, b) => null).then((_) {});
+  });
+
   test('contains', () {
     stream.contains((_) => true).then((_) {});
   });