Remove AsyncError with Expando.

tests/standalone/io/echo_server_stream_test.dart

Index: tests/standalone/io/echo_server_stream_test.dart
diff --git a/tests/standalone/io/echo_server_stream_test.dart b/tests/standalone/io/echo_server_stream_test.dart
index 6eaf05cefa8f4de98cbbb294dcfdf08ff7a88c9b..4c00422af2dc62515b70f8340c8467c02c850189 100644
--- a/tests/standalone/io/echo_server_stream_test.dart
+++ b/tests/standalone/io/echo_server_stream_test.dart
@@ -57,7 +57,10 @@ class EchoServerGame {
     }
 
     void errorHandler(e) {
-      Expect.fail("Socket error $e");
+      String msg = "Socket error $e";
+      var trace = getAttachedStackTrace(e);
+      if (trace != null) msg += "\nStackTrace: $trace";
+      Expect.fail(msg);
     }
 
     void connectHandler() {
@@ -128,7 +131,10 @@ class EchoServer extends TestingServer {
     }
 
     void errorHandler(e) {
-      Expect.fail("Socket error $e");
+      String msg = "Socket error $e";
+      var trace = getAttachedStackTrace(e);
+      if (trace != null) msg += "\nStackTrace: $trace";
+      Expect.fail(msg);
     }
 
     connection.listen(dataReceived, onError: errorHandler);