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

pkg/http/lib/src/byte_stream.dart

Index: pkg/http/lib/src/byte_stream.dart
diff --git a/pkg/http/lib/src/byte_stream.dart b/pkg/http/lib/src/byte_stream.dart
index b53b8eb601fbbfbe563d4af3523ec9ba8cb50ae0..f2c6a0bcaa036547b1bc51dbb5abc7ea29d2b5b4 100644
--- a/pkg/http/lib/src/byte_stream.dart
+++ b/pkg/http/lib/src/byte_stream.dart
@@ -23,7 +23,7 @@ class ByteStream extends StreamView<List<int>> {
   /// Collects the data of this stream in a [Uint8List].
   Future<Uint8List> toBytes() {
     /// TODO(nweiz): use BufferList when issue 6409 is fixed.
-    return reduce(<int>[], (buffer, chunk) {
+    return fold(<int>[], (buffer, chunk) {
       buffer.addAll(chunk);
       return buffer;
     }).then(toUint8List);