Use --trace-service to help diagnose flaky tests.

runtime/observatory/tests/service/pause_on_start_then_step_test.dart

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 // VMOptions=--error_on_bad_type --error_on_bad_override
 
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 import 'dart:async';
 
@@ skipping 68 matching lines @@
          equals(isolate.pauseEvent.timestamp.millisecondsSinceEpoch));
 
  expect(pausetime2.millisecondsSinceEpoch,
         greaterThan(pausetime1.millisecondsSinceEpoch));
 },
 
 ];
 
 main(args) => runIsolateTests(args, tests,
                               testeeConcurrent: testMain,
-                              pause_on_start: true, pause_on_exit: true);
+                              pause_on_start: true,
+                              pause_on_exit: true,
+                              trace_service: true);