anchor.diff:  }
   
   class CompileJSCommand extends DartdevCommand<int> {
  -  CompileJSCommand() : super('js', 'Compile Dart to JavaScript') {
  +  CompileJSCommand() : super('js', 'Compile Dart to JavaScript.') {
       argParser
         ..addOption(
           commonOptions['outputFile'].flag,
  
positive.code:   FutureOr<int> run() async {
      // We expect a single rest argument; the dart entry point.
      if (argResults.rest.length != 1) {
        log.stderr('Missing Dart entry point.');
        printUsage();
        return compileErrorExitCode;
      }
      final String sourcePath = argResults.rest[0];
      if (!checkFile(sourcePath)) {
        return -1;
  
negative.code:     addSubcommand(CompileJSCommand());
      addSubcommand(CompileSnapshotCommand(
        commandName: 'jit-snapshot',
        help: 'to a JIT snapshot.',
        fileExt: 'jit',
        formatName: 'app-jit',
      ));
      addSubcommand(CompileSnapshotCommand(
        commandName: 'kernel',
        help: 'to a kernel snapshot.',
        fileExt: 'dill',
        formatName: 'kernel',
      ));
      addSubcommand(CompileNativeCommand(
        commandName: 'exe',
        help: 'to a self-contained executable.',
        format: 'exe',
      ));
      addSubcommand(CompileNativeCommand(
        commandName: 'aot-snapshot',
        help: 'to an AOT snapshot.',
        format: 'aot',
      ));
    }
  