Rip out the last dart:io dependency from pkg/http.

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:convert';
 import 'dart:typed_data';
 
 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() {
     var completer = new Completer();
-    var sink = new ByteConversionSink.withCallback(completer.complete);
+    var sink = new ByteConversionSink.withCallback((bytes) =>
+        completer.complete(new Uint8List.fromList(bytes)));
     listen(sink.add, onError: completer.completeError, onDone: sink.close,
         cancelOnError: true);
     return completer.future;
   }
 
   /// Collect the data of this stream in a [String], decoded according to
   /// [encoding], which defaults to `UTF8`.
   Future<String> bytesToString([Encoding encoding=UTF8]) =>
       encoding.decodeStream(this);
 
   Stream<String> toStringStream([Encoding encoding=UTF8]) =>
       transform(encoding.decoder);
 }