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

tests/standalone/io/https_client_certificate_test.dart

Index: tests/standalone/io/https_client_certificate_test.dart
diff --git a/tests/standalone/io/https_client_certificate_test.dart b/tests/standalone/io/https_client_certificate_test.dart
index 1066c01aab07fd391c3f0f2f2622a42b9935714d..62144046cab2f263239414e4a407d1277d46eb3d 100644
--- a/tests/standalone/io/https_client_certificate_test.dart
+++ b/tests/standalone/io/https_client_certificate_test.dart
@@ -31,8 +31,8 @@ Function test() {
         .then((response) {
           Expect.equals('CN=localhost', response.certificate.subject);
           Expect.equals('CN=myauthority', response.certificate.issuer);
-          return response.reduce(<int>[],
-                                 (message, data) => message..addAll(data));
+          return response.fold(<int>[],
+                               (message, data) => message..addAll(data));
         })
         .then((message) {
           String received = new String.fromCharCodes(message);