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

sdk/lib/io/websocket_impl.dart

Index: sdk/lib/io/websocket_impl.dart
diff --git a/sdk/lib/io/websocket_impl.dart b/sdk/lib/io/websocket_impl.dart
index 2d29a5f658dd1c0dee32a68a58f14b5a3840ba23..bd21220f69830e9179419dd6ce6a5421bf7d9652 100644
--- a/sdk/lib/io/websocket_impl.dart
+++ b/sdk/lib/io/websocket_impl.dart
@@ -683,9 +683,9 @@ class _WebSocketImpl extends Stream implements WebSocket {
     }
     assert(index == headerSize);
     try {
-      _socket.writeBytes(header);
+      _socket.add(header);
       if (data != null) {
-        _socket.writeBytes(data);
+        _socket.add(data);
       }
     } catch (_) {
       // The socket can be closed before _socket.done have a chance