Fix sdk/lib/io errors found by the analyzer.

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 cb6d099d154f35f658621f214bd519058fb35ad1..6a626092fc4f46843f6496721d298c8698ac3551 100644
--- a/sdk/lib/io/websocket_impl.dart
+++ b/sdk/lib/io/websocket_impl.dart
@@ -898,7 +898,8 @@ class _WebSocketImpl extends Stream implements WebSocket {
   String get closeReason => _closeReason;
 
   void add(data) => _sink.add(data);
-  void addError(error) => _sink.addError(error);
+  void addError(error, [StackTrace stackTrace]) =>
+      _sink.addError(error, stackTrace);
   Future addStream(Stream stream) => _sink.addStream(stream);
   Future get done => _sink.done;