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

tests/standalone/io/http_keep_alive_test.dart

Index: tests/standalone/io/http_keep_alive_test.dart
diff --git a/tests/standalone/io/http_keep_alive_test.dart b/tests/standalone/io/http_keep_alive_test.dart
index 60bb8dea4264bd616af2f2759b8c4dcda14d284e..c2160d3db075968ff7b27e79564742ed4a0d1d4a 100644
--- a/tests/standalone/io/http_keep_alive_test.dart
+++ b/tests/standalone/io/http_keep_alive_test.dart
@@ -29,7 +29,7 @@ Future<HttpServer> startServer() {
       int length = int.parse(request.queryParameters["length"]);
       var buffer = new List<int>.filled(length, 0);
       if (!chunked) request.response.contentLength = length;
-      request.response.writeBytes(buffer);
+      request.response.add(buffer);
       request.response.close();
     });
     return server;