Remove Sink and move CollectionSink to async.

pkg/http/lib/src/streamed_request.dart

 // Copyright (c) 2012, 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 streamed_request;
 
 import 'dart:async';
 import 'dart:io';
 import 'dart:uri';
 
 import 'byte_stream.dart';
 import 'base_request.dart';
 
 /// An HTTP request where the request body is sent asynchronously after the
 /// connection has been established and the headers have been sent.
 ///
 /// When the request is sent via [BaseClient.send], only the headers and
 /// whatever data has already been written to [StreamedRequest.stream] will be
 /// sent immediately. More data will be sent as soon as it's written to
 /// [StreamedRequest.sink], and when the sink is closed the request will end.
 class StreamedRequest extends BaseRequest {
   /// The sink to which to write data that will be sent as the request body.
   /// This may be safely written to before the request is sent; the data will be
   /// buffered.
   ///
   /// Closing this signals the end of the request.
-  Sink<List<int>> get sink => _controller.sink;
+  StreamSink<List<int>> get sink => _controller.sink;
 
   /// The controller for [sink], from which [BaseRequest] will read data for
   /// [finalize].
   final StreamController<List<int>> _controller;
 
   /// Creates a new streaming request.
   StreamedRequest(String method, Uri url)
     : super(method, url),
       _controller = new StreamController<List<int>>();
 
   /// Freezes all mutable fields other than [stream] and returns a
   /// single-subscription [ByteStream] that emits the data being written to
   /// [sink].
   ByteStream finalize() {
     super.finalize();
     return new ByteStream(_controller.stream);
   }
 }