Make IOSink implement StringSink

tests/standalone/io/raw_secure_socket_pause_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";
 import "dart:io";
 import "dart:isolate";
 
 Future<HttpServer> startServer() {
   return HttpServer.bindSecure(
       "127.0.0.1",
       0,
       backlog: 5,
       certificateName: 'localhost_cert').then((server) {
     server.listen((HttpRequest request) {
       request.listen(
         (_) { },
         onDone: () {
           request.response.contentLength = 100;
           for (int i = 0; i < 10; i++) {
-            request.response.add([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
+            request.response.writeBytes([0, 1, 2, 3, 4, 5, 6, 7, 8, 9]);
           }
           request.response.close();
         });
     });
     return server;
   });
 }
 
 void InitializeSSL() {
   var testPkcertDatabase =
@@ skipping 61 matching lines @@
                 break;
               default: throw "Unexpected event $event";
             }
           },
           onError: (AsyncError a) {
             Expect.fail("onError handler of RawSecureSocket stream hit: $a");
           });
     });
   });
 }