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

tests/standalone/io/http_keep_alive_test.dart

Index: tests/standalone/io/http_keep_alive_test.dart
diff --git a/tests/standalone/io/http_keep_alive_test.dart b/tests/standalone/io/http_keep_alive_test.dart
index 30373e8c10681c8a02535953b542d8b7745c680b..ad20ea8af396407922ffb851e5d76c1de932a6da 100644
--- a/tests/standalone/io/http_keep_alive_test.dart
+++ b/tests/standalone/io/http_keep_alive_test.dart
@@ -14,7 +14,7 @@ Future getData(HttpClient client, int port, bool chunked, int length) {
   return client.get("localhost", port, "/?chunked=$chunked&length=$length")
       .then((request) => request.close())
       .then((response) {
-        return response.reduce(0, (bytes, data) => bytes + data.length)
+        return response.fold(0, (bytes, data) => bytes + data.length)
             .then((bytes) {
               Expect.equals(length, bytes);
             });