server_test.dart: server DartServicesClient assists (failed)

We're seeing failures from:

```
test/presubmit/server_test.dart: server DartServicesClient assists (failed)

Expected: non-empty
    Actual: []
  
  package:matcher                                      expect
  test/probes_and_presubmit/server_testing.dart 311:7  testServer.<fn>.<fn>
```

on the stable channel.