Ignore GC events in websocket_client_test

tests/standalone/vmservice/websocket_client_test.dart

Index: tests/standalone/vmservice/websocket_client_test.dart
diff --git a/tests/standalone/vmservice/websocket_client_test.dart b/tests/standalone/vmservice/websocket_client_test.dart
index e6d978232dccff7bdcd1c4a4ee934e13190d1f62..25b94aee08cff968c4e9de9478990ce4ac37b620 100644
--- a/tests/standalone/vmservice/websocket_client_test.dart
+++ b/tests/standalone/vmservice/websocket_client_test.dart
@@ -12,6 +12,10 @@ class ClientsRequestTest extends ServiceWebSocketRequestHelper {
   int _count = 0;
 
   onResponse(var seq, Map response) {
+    if (seq == null) {
+      // Ignore push events.
+      return;
+    }
     if (seq == 'cli') {
       // Verify response is correct for 'cli' sequence id.
       Expect.equals('ClientList', response['type']);