anchor.diff:          StdioProcess process;
           try {
             var args = <String>[];
  +          if (experimentalFlags[ExperimentalFlag.nonNullable]) {
  +            args.add("--null-safety");
  +          }
             args.add(generated.path);
             process = await StdioProcess.run(context.vm.toFilePath(), args);
             print(process.output);
  
positive.code:     show CompilerOptions, DiagnosticMessage;
  
  import 'package:front_end/src/api_prototype/experimental_flags.dart'
      show ExperimentalFlag;
  
  import 'package:front_end/src/api_prototype/standard_file_system.dart'
      show StandardFileSystem;
  
negative.code: 
    Map<ExperimentalFlag, bool> computeExperimentalFlags(
        Map<ExperimentalFlag, bool> forcedExperimentalFlags) {
      Map<ExperimentalFlag, bool> flags = new Map.from(defaultExperimentalFlags);
      flags.addAll(_experimentalFlags);
      flags.addAll(forcedExperimentalFlags);
      return flags;
    }
  