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

tests/standalone/io/http_compression_test.dart

Index: tests/standalone/io/http_compression_test.dart
diff --git a/tests/standalone/io/http_compression_test.dart b/tests/standalone/io/http_compression_test.dart
index ce3f9c9fe1e3dffc7dfe0cd09eb4d67f359e1f8e..c347023d1b053891085b86ef853ae37c23a17d41 100644
--- a/tests/standalone/io/http_compression_test.dart
+++ b/tests/standalone/io/http_compression_test.dart
@@ -28,7 +28,7 @@ void testServerCompress() {
                           response.headers.value(HttpHeaders.CONTENT_ENCODING));
             response
                 .transform(new ZLibInflater())
-                .reduce([], (list, b) {
+                .fold([], (list, b) {
                   list.addAll(b);
                   return list;
                 }).then((list) {