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

pkg/scheduled_test/test/descriptor/utils.dart

Index: pkg/scheduled_test/test/descriptor/utils.dart
diff --git a/pkg/scheduled_test/test/descriptor/utils.dart b/pkg/scheduled_test/test/descriptor/utils.dart
index 2c48ab483b6fa6834448ae36480b19c71e698293..bd9ae57ed3f4eee23db4033b9d41650106e0e55a 100644
--- a/pkg/scheduled_test/test/descriptor/utils.dart
+++ b/pkg/scheduled_test/test/descriptor/utils.dart
@@ -32,7 +32,7 @@ void scheduleSandbox() {
 }
 
 Future<List<int>> byteStreamToList(Stream<List<int>> stream) {
-  return stream.reduce(<int>[], (buffer, chunk) {
+  return stream.fold(<int>[], (buffer, chunk) {
     buffer.addAll(chunk);
     return buffer;
   });