anchor.diff:        help: "Replace analyzer error expectations.", negatable: false);
     parser.addFlag("update-cfe",
         help: "Replace CFE error expectations.", negatable: false);
  +
  +  parser.addSeparator("Other flags:");
  +  parser.addFlag("nnbd",
  +      help: "Analyze with the 'non-nullable' experiment enabled.",
  +      negatable: false);
   
     var results = parser.parse(args);
   
  
positive.code:       results["update-cfe"] as bool ||
        results["update"] as bool;
  
    if (!removeAnalyzer && !removeCfe && !insertAnalyzer && !insertCfe) {
      _usageError(
          parser, "Must provide at least one flag for an operation to perform.");
  
negative.code:     bool removeAnalyzer,
      bool removeCfe,
      bool insertAnalyzer,
      bool insertCfe,
      bool nnbd}) async {
    stdout.write("${file.path}...");
    var source = file.readAsStringSync();
    var testFile = TestFile.parse(Path("."), file.absolute.path, source);
  
    var experiments = [
      if (nnbd) "non-nullable",
      if (testFile.experiments.isNotEmpty) ...testFile.experiments
    ];
  
    var options = [
      ...testFile.sharedOptions,
      if (experiments.isNotEmpty) "--enable-experiment=${experiments.join(',')}"
    ];
  
    var errors = <StaticError>[];
    if (insertAnalyzer) {
  