anchor.diff:  
     /// The callback that's run when [Directory.watch] emits a batch of events.
     void _onBatch(List<FileSystemEvent> batch) {
  +    if (_runningOnBuildbot) {
  +      print("======== batch:");
  +      for (var event in batch) {
  +        print("  ${_formatEvent(event)}");
  +      }
  +
  +      print("known files:");
  +      for (var foo in _files.toSet()) {
  +        print("  ${p.relative(foo, from: directory)}");
  +      }
  +    }
  +
       batches++;
   
       _sortEvents(batch).forEach((path, events) {
  +      var relativePath = p.relative(path, from: directory);
  +      if (_runningOnBuildbot) {
  +        print("events for $relativePath:\n");
  +        for (var event in events) {
  +          print("  ${_formatEvent(event)}");
  +        }
  +      }
  +
         var canonicalEvent = _canonicalEvent(events);
         events = canonicalEvent == null ?
             _eventsBasedOnFileSystem(path) : [canonicalEvent];
  +      if (_runningOnBuildbot) {
  +        print("canonical event for $relativePath: "
  +            "${_formatEvent(canonicalEvent)}");
  +        print("actionable events for $relativePath: "
  +            "${events.map(_formatEvent)}");
  +      }
   
         for (var event in events) {
           if (event is FileSystemCreateEvent) {
  
positive.code:     var fileExists = new File(path).existsSync();
      var dirExists = new Directory(path).existsSync();
  
      var events = [];
      if (fileExisted) {
        if (fileExists) {
  
negative.code:     }, cancelOnError: cancelOnError);
      _subscriptions.add(subscription);
    }
  
    /// Return a human-friendly string representation of [event].
    String _formatEvent(FileSystemEvent event) {
      if (event == null) return 'null';
  
      var path = p.relative(event.path, from: directory);
      var type = event.isDirectory ? 'directory' : 'file';
      if (event is FileSystemCreateEvent) {
        return "create $type $path";
      } else if (event is FileSystemDeleteEvent) {
        return "delete $type $path";
      } else if (event is FileSystemModifyEvent) {
        return "modify $type $path";
      } else if (event is FileSystemMoveEvent) {
        return "move $type $path to "
            "${p.relative(event.destination, from: directory)}";
      }
    }
  
    // TODO(nweiz): remove this when the buildbots calm down.
    /// Whether this code is running on a buildbot.
    bool get _runningOnBuildbot =>
      Platform.environment['LOGNAME'] == 'chrome-bot';
  }
  