anchor.diff:  
       if (entry is pkg_file.File) {
         await _processFile(entry,
  -          dryRun: dryRun,
  -          remove: removeSources,
  -          insert: insertSources,
  -          nullSafety: nullSafety);
  +          dryRun: dryRun, remove: removeSources, insert: insertSources);
       }
     }
   }
  
positive.code: }
  
  Future<void> _processFile(File file,
      {bool dryRun,
      Set<ErrorSource> remove,
      Set<ErrorSource> insert,
      bool nullSafety}) async {
    stdout.write("${file.path}...");
    var source = file.readAsStringSync();
    var testFile = TestFile.parse(Path("."), file.absolute.path, source);
  
    var experiments = [
      if (nullSafety) "non-nullable",
      if (testFile.experiments.isNotEmpty) ...testFile.experiments
    ];
  
  
negative.code:   // tell which web errors are web-specific.
    List<StaticError> cfeErrors;
    if (insert.contains(ErrorSource.cfe) || insert.contains(ErrorSource.web)) {
      var cfeOptions = [
        if (testFile.requirements.contains(Feature.nnbdWeak)) "--nnbd-weak",
        if (testFile.requirements.contains(Feature.nnbdStrong)) "--nnbd-strong",
        ...options
      ];
      // Clear the previous line.
      stdout.write("\r${file.path}                      ");
      stdout.write("\r${file.path} (Running CFE...)");
      cfeErrors = await runCfe(file.absolute.path, cfeOptions);
      if (cfeErrors == null) {
        print("Error: failed to update ${file.path}");
      } else if (insert.contains(ErrorSource.cfe)) {
  