Make IOSink implement StringSink

tests/standalone/io/secure_session_resume_test.dart

Index: tests/standalone/io/secure_session_resume_test.dart
diff --git a/tests/standalone/io/secure_session_resume_test.dart b/tests/standalone/io/secure_session_resume_test.dart
index f862c649f0fe9c707d61ee8427b0d3bdf6e5b823..97f0adeb4c3489662334cd915f40085dae2e5260 100644
--- a/tests/standalone/io/secure_session_resume_test.dart
+++ b/tests/standalone/io/secure_session_resume_test.dart
@@ -34,7 +34,7 @@ Future<SecureServerSocket> startServer() {
             String received = new String.fromCharCodes(message);
             Expect.isTrue(received.contains("Hello from client "));
             String name = received.substring(received.indexOf("client ") + 7);
-            client.add("Welcome, client $name".codeUnits);
+            client.write("Welcome, client $name");
             client.close();
           });
     });
@@ -44,7 +44,7 @@ Future<SecureServerSocket> startServer() {
 
 Future testClient(server, name) {
   return SecureSocket.connect(HOST_NAME, server.port).then((socket) {
-    socket.add("Hello from client $name".codeUnits);
+    socket.write("Hello from client $name");
     socket.close();
     return socket.reduce(<int>[], (message, data) => message..addAll(data))
         .then((message) {