Fix vm_timeline_flags_test flake

runtime/observatory/tests/service/vm_timeline_flags_test.dart

 // Copyright (c) 2016, 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 'dart:developer';
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'service_test_common.dart';
 import 'test_helper.dart';
@@ skipping 45 matching lines @@
   (Isolate isolate) async {
     // Get the flags.
     Map flags = await isolate.vm.invokeRpcNoUpgrade('_getVMTimelineFlags', {});
     expect(flags['type'], 'TimelineFlags');
     // Confirm that only Dart is being recorded.
     expect(flags['recordedStreams'].length, equals(1));
     expect(flags['recordedStreams'].contains('Dart'), isTrue);
     print(flags);
   },
   resumeIsolate,
+  hasStoppedAtBreakpoint,
   (Isolate isolate) async {
     // Get the timeline.
     Map result = await isolate.vm.invokeRpcNoUpgrade('_getVMTimeline', {});
     expect(result['type'], equals('_Timeline'));
     expect(result['traceEvents'], new isInstanceOf<List>());
     print(result['traceEvents']);
     // Confirm that Dart events are added.
     expect(filterEvents(result['traceEvents'], isDart).length, greaterThan(0));
     // Confirm that zero non-Dart events are added.
     expect(
@@ skipping 13 matching lines @@
     dartEventCount = filterEvents(result['traceEvents'], isDart).length;
   },
   (Isolate isolate) async {
     // Get the flags.
     Map flags = await isolate.vm.invokeRpcNoUpgrade('_getVMTimelineFlags', {});
     expect(flags['type'], 'TimelineFlags');
     // Confirm that 'Dart' is not being recorded.
     expect(flags['recordedStreams'].length, equals(0));
   },
   resumeIsolate,
+  hasStoppedAtBreakpoint,
   (Isolate isolate) async {
     // Grab the timeline and verify that we haven't added any new Dart events.
     Map result = await isolate.vm.invokeRpcNoUpgrade('_getVMTimeline', {});
     expect(result['type'], equals('_Timeline'));
     expect(result['traceEvents'], new isInstanceOf<List>());
     expect(filterEvents(result['traceEvents'], isDart).length, dartEventCount);
     // Confirm that zero non-Dart events are added.
     expect(
         filterEvents(result['traceEvents'], isNotDartAndMetaData).length,
         equals(0));
   },
 ];
 
 main(args) async => runIsolateTests(args,
                                     tests,
                                     testeeConcurrent: primeDartTimeline);