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

utils/pub/io.dart

Index: utils/pub/io.dart
diff --git a/utils/pub/io.dart b/utils/pub/io.dart
index a0e69a9172ee038144b4bb04d82094bbdd9144b6..3eed22c0aa971b3a7e3c35dc7221df5348ebe2f2 100644
--- a/utils/pub/io.dart
+++ b/utils/pub/io.dart
@@ -328,7 +328,7 @@ Future<bool> confirm(String message) {
 /// Reads and discards all output from [stream]. Returns a [Future] that
 /// completes when the stream is closed.
 Future drainStream(Stream stream) {
-  return stream.reduce(null, (x, y) {});
+  return stream.fold(null, (x, y) {});
 }
 
 /// Returns a [EventSink] that pipes all data to [consumer] and a [Future] that