anchor.diff:    }
   
     // Save the durations Map to a file.
  -  final String destinationDirectory = 'build';
  -  await _fs.directory(destinationDirectory).create(recursive: true);
  -  final File file = _fs.file(path.join(destinationDirectory, 'transition_durations.timeline.json'));
  +  final File file = await _fs.file(outputPath).create(recursive: true);
     await file.writeAsString(new JsonEncoder.withIndent('  ').convert(durations));
   }
   
  
positive.code:       TimelineSummary summary = new TimelineSummary.summarize(timeline);
        await summary.writeSummaryToFile('transitions', pretty: true);
        try {
          await saveDurationsHistogram(timeline.json['traceEvents']);
        } catch(_) {
          await summary.writeTimelineToFile('transitions', pretty: true);
          print('ERROR: failed to extract transition events. Here is the full timeline:\n');
          print(await _fs.file('build/transitions.timeline.json').readAsString());
          rethrow;
        }
      }, timeout: new Timeout(new Duration(minutes: 5)));
  
negative.code: 
  final FileSystem _fs = new LocalFileSystem();
  
  Future<Null> saveDurationsHistogram(List<Map<String, dynamic>> events, String outputPath) async {
    final Map<String, List<int>> durations = new Map<String, List<int>>();
    Map<String, dynamic> startEvent;
  
  