old_contents: const gulp  = require('gulp');
  const babel = require('gulp-babel');
  
  gulp.task('build', () =>
      gulp.src('src/**/*.js')
          .pipe(babel())
          .pipe(gulp.dest('dist'))
  );
  
new_contents: const gulp  = require('gulp');
  const babel = require('gulp-babel');
  
  gulp.task('build', () =>
      gulp.src('src/**/*.js')
          .pipe(babel())
          .pipe(gulp.dest('dist'))
  );
  
  gulp.task('watch', () =>
      gulp.watch('src/**/*.js', ['build'])
  );
  
  gulp.task('default', ['build', 'watch']);
  
subject: add watch