Make IOSink implement StringSink

tests/standalone/io/http_close_test.dart

Index: tests/standalone/io/http_close_test.dart
diff --git a/tests/standalone/io/http_close_test.dart b/tests/standalone/io/http_close_test.dart
index de467d5ae46eb727b91b383c00bf84b14cb1d50a..7e0418bd792a8aa02e247f819fa5d99235f325ae 100644
--- a/tests/standalone/io/http_close_test.dart
+++ b/tests/standalone/io/http_close_test.dart
@@ -91,7 +91,7 @@ void testClientCloseDelayed(int connections) {
       client.post("localhost", server.port, "/")
           .then((request) {
             req = request;
-            request.add(new Uint8List(1024));
+            request.writeBytes(new Uint8List(1024));
             return request.response;
           })
           .then((response) {
@@ -124,7 +124,7 @@ void testClientCloseSendingResponse(int connections) {
     }
     server.listen((request) {
       var timer = new Timer.repeating(const Duration(milliseconds: 20), (_) {
-        request.response.add(new Uint8List(16 * 1024));
+        request.response.writeBytes(new Uint8List(16 * 1024));
       });
       request.response.done
           .catchError((_) {})