Make IOSink implement StringSink

tests/standalone/io/echo_server_stream_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.
 //
 // Echo server test program to test socket streams.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // VMOptions=--short_socket_write
 // VMOptions=--short_socket_read --short_socket_write
@@ skipping 45 matching lines @@
     }
 
     void errorHandler(e) {
       Expect.fail("Socket error $e");
     }
 
     void connectHandler() {
       _socket.listen(onData,
                      onError: errorHandler,
                      onDone: onClosed);
-      _socket.add(_buffer);
+      _socket.writeBytes(_buffer);
       _socket.close();
       data = new List<int>(MSGSIZE);
     }
 
     Socket.connect(TestingServer.HOST, _port).then((s) {
       _socket = s;
       connectHandler();
     });
   }
 
@@ skipping 36 matching lines @@
     void dataReceived(List<int> data) {
       int bytesRead;
       bytesRead = data.length;
       if (bytesRead > 0) {
         buffer.setRange(offset, data.length, data);
         offset += bytesRead;
         for (int i = 0; i < offset; i++) {
           Expect.equals(EchoServerGame.FIRSTCHAR + i, buffer[i]);
         }
         if (offset == MSGSIZE) {
-          connection.add(buffer);
+          connection.writeBytes(buffer);
           connection.close();
         }
       }
     }
 
     void errorHandler(e) {
       Expect.fail("Socket error $e");
     }
 
     connection.listen(dataReceived, onError: errorHandler);
   }
 }
 
 main() {
   EchoServerGame echoServerGame = new EchoServerGame.start();
 }