Remove AsyncError with Expando.

tests/standalone/io/socket_many_connections_test.dart

Index: tests/standalone/io/socket_many_connections_test.dart
diff --git a/tests/standalone/io/socket_many_connections_test.dart b/tests/standalone/io/socket_many_connections_test.dart
index b10ad7179cb1158557f67f819bd913c4932dc2d5..c6c557ef5d1ab98d720cdb7686401986102b9254 100644
--- a/tests/standalone/io/socket_many_connections_test.dart
+++ b/tests/standalone/io/socket_many_connections_test.dart
@@ -80,7 +80,10 @@ class TestServer extends TestingServer {
     }
 
     void errorHandler(e) {
-      print("Socket error $e");
+      String msg = "Socket error $e";
+      var trace = getAttachedStackTrace(e);
+      if (trace != null) msg += "\nStackTrace: $trace";
+      print(msg);
       connection.close();
     }