Make IOSink implement StringSink

tests/standalone/io/echo_server_stream_test.dart

Index: tests/standalone/io/echo_server_stream_test.dart
diff --git a/tests/standalone/io/echo_server_stream_test.dart b/tests/standalone/io/echo_server_stream_test.dart
index b3a18689b33a40f5e5171a32f9ce32a3dd1b9c87..6cd0f37cdc229dd5f339cd948a83227a9950a55e 100644
--- a/tests/standalone/io/echo_server_stream_test.dart
+++ b/tests/standalone/io/echo_server_stream_test.dart
@@ -63,7 +63,7 @@ class EchoServerGame {
       _socket.listen(onData,
                      onError: errorHandler,
                      onDone: onClosed);
-      _socket.add(_buffer);
+      _socket.writeBytes(_buffer);
       _socket.close();
       data = new List<int>(MSGSIZE);
     }
@@ -120,7 +120,7 @@ class EchoServer extends TestingServer {
           Expect.equals(EchoServerGame.FIRSTCHAR + i, buffer[i]);
         }
         if (offset == MSGSIZE) {
-          connection.add(buffer);
+          connection.writeBytes(buffer);
           connection.close();
         }
       }