Remove deprecated parts of dart:async.

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 d6ddcbdb05751419aa54dfee40ea52459073a07f..0c789ec850b9f5677670ddbcd344b2ecc2ae5875 100644
--- a/tests/standalone/io/socket_many_connections_test.dart
+++ b/tests/standalone/io/socket_many_connections_test.dart
@@ -81,9 +81,8 @@ class TestServer extends TestingServer {
       connection.close();
     }
 
-    void errorHandler(e) {
+    void errorHandler(e, trace) {
       String msg = "Socket error $e";
-      var trace = getAttachedStackTrace(e);
       if (trace != null) msg += "\nStackTrace: $trace";
       print(msg);
       connection.close();