anchor.diff:    Timer _knownFilesSetChangesTimer;
   
     /**
  +   * Whether the [knownFilesSetChanges] stream is requested.
  +   */
  +  bool _knownFilesSetChangesRequested = false;
  +
  +  /**
      * The controller for the [knownFilesSetChanges] stream.
      */
     final StreamController<KnownFilesSetChange> _knownFilesSetChangesController =
  
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';
  
  /**
  