Make IOSink implement StringSink

tests/standalone/io/http_close_test.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.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
 
 import "dart:async";
@@ skipping 73 matching lines @@
     server.listen((request) {
       request.pipe(request.response)
           .then((_) => check());
     });
     var client = new HttpClient();
     for (int i = 0; i < connections; i++) {
       var req;
       client.post("localhost", server.port, "/")
           .then((request) {
             req = request;
-            request.add(new Uint8List(1024));
+            request.writeBytes(new Uint8List(1024));
             return request.response;
           })
           .then((response) {
             req.close();
             // Ensure we don't accept the response until we have send the entire
             // request.
             response.listen(
                 (_) {},
                 onDone: () {
                   check();
@@ skipping 12 matching lines @@
       // Wait for both server and client to see the connections as closed.
       if (closed == connections * 2) {
         Expect.equals(0, server.connectionsInfo().active);
         Expect.equals(server.connectionsInfo().total,
                       server.connectionsInfo().idle);
         server.close();
       }
     }
     server.listen((request) {
       var timer = new Timer.repeating(const Duration(milliseconds: 20), (_) {
-        request.response.add(new Uint8List(16 * 1024));
+        request.response.writeBytes(new Uint8List(16 * 1024));
       });
       request.response.done
           .catchError((_) {})
           .whenComplete(() {
             check();
             timer.cancel();
           });
     });
     var client = new HttpClient();
     for (int i = 0; i < connections; i++) {
@@ skipping 13 matching lines @@
 }
 
 
 void main() {
   testClientAndServerCloseNoListen(10);
   testClientCloseServerListen(10);
   testClientCloseDelayed(10);
   testClientCloseSendingResponse(10);
 }