Make IOSink implement StringSink

tests/standalone/io/secure_server_socket_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 152 matching lines @@
       int bytesWritten = 0;
       List<int> data = new List<int>(messageSize);
 
       client.listen(
         (buffer) {
           Expect.isTrue(bytesWritten == 0);
           data.setRange(bytesRead, buffer.length, buffer);
           bytesRead += buffer.length;
           if (bytesRead == data.length) {
             verifyTestData(data);
-            client.add(data);
+            client.writeBytes(data);
             client.close();
           }
         },
         onDone: () {
           server.close();
         });
     });
 
     SecureSocket.connect(HOST_NAME, server.port).then((socket) {
       int bytesRead = 0;
       int bytesWritten = 0;
       List<int> dataSent = createTestData();
       List<int> dataReceived = new List<int>(dataSent.length);
-      socket.add(dataSent);
+      socket.writeBytes(dataSent);
       socket.close();  // Can also be delayed.
       socket.listen(
         (List<int> buffer) {
           dataReceived.setRange(bytesRead, buffer.length, buffer);
           bytesRead += buffer.length;
         },
         onDone: () {
           verifyTestData(dataReceived);
           socket.close();
           port.close();
@@ skipping 11 matching lines @@
   testArguments();
   testSimpleBind();
   testInvalidBind();
   testSimpleConnect(CERTIFICATE);
   testSimpleConnect("CN=localhost");
   testSimpleConnectFail("not_a_nickname");
   testSimpleConnectFail("CN=notARealDistinguishedName");
   testServerListenAfterConnect();
   testSimpleReadWrite();
 }