Ejemplo Gulp con BrowserSync y ES6+

gulpfile.js

const gulp = require('gulp');
const browserify = require('browserify');
const babelify = require('babelify');
const concat = require('gulp-concat');
const concatcss = require('gulp-concat-css');
const browserSync = require('browser-sync');
const babel = require('gulp-babel');
const postcss = require('gulp-postcss');
const cssnext = require('postcss-cssnext');
const postcssImport = require('postcss-import');
const postcssNested = require('postcss-nested');
const postcssVars = require('postcss-custom-properties');
const fs = require('fs');

gulp.task('serve', ['styles', 'scripts'], () => {
  browserSync({
    server: {
      baseDir: './'
    }
  });

  gulp.watch('css/*.css', ['styles']);
  gulp.watch('js/*.es', ['scripts']);
  gulp.watch("index.html").on('change', browserSync.reload);
});

gulp.task('scripts', () => {
  return (
    browserify()
      .transform(babelify)
      .require('./js/index.es', { entry: true})
      .bundle()
      .pipe(fs.createWriteStream('bundle.js'))
    /*gulp
      .src('js/*.es')
      .pipe(babel())
      .pipe(concat('bundle.js'))
      .pipe(gulp.dest('./'))*/
  );
});

gulp.task('styles', () => {
  const plugins = [
    cssnext(),
    postcssImport(),
    postcssNested(),
    postcssVars()
  ];

  return (
    gulp
      .src('css/*.css')
      .pipe(postcss(plugins))
      .pipe(concatcss('bundle.css'))
      .pipe(gulp.dest('./'))
      .pipe(browserSync.stream())
  );
});

gulp.task('default', ['serve']);

package.json

{
  "name": "clicklead-dash",
  "version": "1.0.0",
  "description": "",
  "main": "gulpfile.js",
  "scripts": {
    "test": "echo \"Error: no test specified\" && exit 1"
  },
  "author": "Gustavo García",
  "license": "MIT",
  "devDependencies": {
    "babel-preset-es2015": "^6.24.1",
    "babel-preset-es2016": "^6.24.1",
    "babel-preset-es2017": "^6.24.1",
    "babelify": "^7.3.0",
    "browser-sync": "^2.18.8",
    "browserify": "^14.3.0",
    "faker": "^4.1.0",
    "gulp": "^3.9.1",
    "gulp-babel": "^6.1.2",
    "gulp-concat": "^2.6.1",
    "gulp-concat-css": "^2.3.0",
    "gulp-postcss": "^6.4.0",
    "postcss-cssnext": "^2.10.0",
    "postcss-custom-properties": "^5.0.2",
    "postcss-import": "^9.1.0",
    "postcss-nested": "^1.0.0"
  }
}