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

pkg/http/lib/src/byte_stream.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library byte_stream;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:typeddata';
 
 import 'utils.dart';
 
 /// A stream of chunks of bytes representing a single piece of data.
 class ByteStream extends StreamView<List<int>> {
   ByteStream(Stream<List<int>> stream)
     : super(stream);
 
   /// Returns a single-subscription byte stream that will emit the given bytes
   /// in a single chunk.
   factory ByteStream.fromBytes(List<int> bytes) =>
     new ByteStream(streamFromIterable([bytes]));
 
   /// 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);
   }
 
   /// Collect the data of this stream in a [String], decoded according to
   /// [encoding], which defaults to `Encoding.UTF_8`.
   Future<String> bytesToString([Encoding encoding=Encoding.UTF_8]) =>
       toBytes().then((bytes) => decodeString(bytes, encoding));
 
   Stream<String> toStringStream([Encoding encoding=Encoding.UTF_8]) =>
       map((bytes) => decodeString(bytes, encoding));
 }