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

tests/standalone/io/http_proxy_test.dart

Index: tests/standalone/io/http_proxy_test.dart
diff --git a/tests/standalone/io/http_proxy_test.dart b/tests/standalone/io/http_proxy_test.dart
index 945189d7198dcab7d8b37574d5ee21094d982c1b..173c3c05493ffc326a8167a650e362bb3a1c02e6 100644
--- a/tests/standalone/io/http_proxy_test.dart
+++ b/tests/standalone/io/http_proxy_test.dart
@@ -28,7 +28,7 @@ class Server {
         var response = request.response;
         requestCount++;
         // Check whether a proxy or direct connection is expected.
-        bool direct = directRequestPaths.reduce(
+        bool direct = directRequestPaths.fold(
             false,
             (prev, path) => prev ? prev : path == request.uri.path);
         if (!direct && proxyHops > 0) {