anchor.diff:     * Return the [Stream] that is periodically notified about changes to the
      * known files set.
      */
  -  Stream<KnownFilesSetChange> get knownFilesSetChanges =>
  -      _knownFilesSetChangesController.stream;
  +  Stream<KnownFilesSetChange> get knownFilesSetChanges {
  +    // If this is the first (and actually the only) time when the stream is
  +    // requested, schedule the timer to send updates.
  +    if (!_knownFilesSetChangesRequested) {
  +      _knownFilesSetChangesRequested = true;
  +      _scheduleKnownFilesSetChange();
  +    }
  +    return _knownFilesSetChangesController.stream;
  +  }
   
     @visibleForTesting
     FileSystemStateTestView get test => _testView;
  
positive.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/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:front_end/src/byte_store/byte_store.dart';
  import 'package:meta/meta.dart';
  
  /**
  
negative.code:   Timer _knownFilesSetChangesTimer;
  
    /**
     * Whether the [knownFilesSetChanges] stream is requested.
     */
    bool _knownFilesSetChangesRequested = false;
  
    /**
     * The controller for the [knownFilesSetChanges] stream.
     */
    final StreamController<KnownFilesSetChange> _knownFilesSetChangesController =
  