Remove StreamSink(replaced by EventSink) and make IOSink extend EventSink.

tests/standalone/io/secure_client_server_test.dart

Index: tests/standalone/io/secure_client_server_test.dart
diff --git a/tests/standalone/io/secure_client_server_test.dart b/tests/standalone/io/secure_client_server_test.dart
index 2034e83d64ca3c83f355b7009d1b376161b5cf8b..6681c1125f489f288d40fc19e69e9eb63f5a4506 100644
--- a/tests/standalone/io/secure_client_server_test.dart
+++ b/tests/standalone/io/secure_client_server_test.dart
@@ -23,7 +23,7 @@ Future<SecureServerSocket> startEchoServer() {
     server.listen((SecureSocket client) {
       client.fold(<int>[], (message, data) => message..addAll(data))
           .then((message) {
-            client.writeBytes(message);
+            client.add(message);
             client.close();
           });
     });