anchor.diff:    }
   
     void _scheduleKnownFilesSetChange() {
  +    // Schedule the timer only if there is a client who listens the stream.
  +    if (!_knownFilesSetChangesRequested) {
  +      return;
  +    }
  +
       Duration delay = _knownFilesSetChangesDelay ?? new Duration(seconds: 1);
       _knownFilesSetChangesTimer ??= new Timer(delay, () {
         Set<String> addedFiles = _addedKnownFiles.toSet();
  
positive.code:    * Return the [Stream] that is periodically notified about changes to the
     * known files set.
     */
    Stream<KnownFilesSetChange> get knownFilesSetChanges =>
        _knownFilesSetChangesController.stream;
  
    @visibleForTesting
    FileSystemStateTestView get test => _testView;
  
negative.code: import 'package:crypto/crypto.dart';
  import 'package:front_end/src/base/api_signature.dart';
  import 'package:front_end/src/base/performace_logger.dart';
  import 'package:front_end/src/byte_store/byte_store.dart';
  import 'package:front_end/src/fasta/builder/builder.dart' as fasta;
  import 'package:front_end/src/fasta/parser/parser.dart' as fasta;
  import 'package:front_end/src/fasta/scanner.dart' as fasta;
  import 'package:meta/meta.dart';
  
  /**
  