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

tests/standalone/io/http_session_test.dart

Index: tests/standalone/io/http_session_test.dart
diff --git a/tests/standalone/io/http_session_test.dart b/tests/standalone/io/http_session_test.dart
index 69f6a6b0d9b0f096099903986311ba635886a182..5648bd0597871a63a18b39e3948ef5a799c05b9b 100644
--- a/tests/standalone/io/http_session_test.dart
+++ b/tests/standalone/io/http_session_test.dart
@@ -8,7 +8,7 @@ import 'dart:io';
 const SESSION_ID = "DARTSESSID";
 
 String getSessionId(List<Cookie> cookies) {
-  var id = cookies.reduce(null, (last, cookie) {
+  var id = cookies.fold(null, (last, cookie) {
     if (last != null) return last;
     if (cookie.name.toUpperCase() == SESSION_ID) {
       Expect.isTrue(cookie.httpOnly);
@@ -30,7 +30,7 @@ Future<String> connectGetSession(
       return request.close();
     })
     .then((response) {
-      return response.reduce(getSessionId(response.cookies), (v, _) => v);
+      return response.fold(getSessionId(response.cookies), (v, _) => v);
     });
 }