Current workflow

.babelrc

{
  "presets": [ "es2015" ]
}

gulpfile.babel.js

import gulp         from 'gulp';
import sass         from 'gulp-sass';
import sassBi       from 'gulp-sass-bulk-import';

import browserSync  from 'browser-sync';

import babel        from 'gulp-babel';

import sourcemaps   from 'gulp-sourcemaps';
import rename       from 'gulp-rename';
import notify       from 'gulp-notify';
import gulpif       from 'gulp-if';
import concat       from 'gulp-concat';
import del          from 'del';
import uglify       from 'gulp-uglify';
import size         from 'gulp-filesize';
import comebineMq   from 'gulp-combine-mq';
import cleanCss     from 'gulp-clean-css';
import imagemin     from 'gulp-imagemin';

import merge        from 'merge-stream';

const GULP_NOTIFY   = true;

const BS_PROXY      = 'tony.dev',
      BS_USE_XIP    = false,
      BS_ONLINE     = true,
      BS_OPEN       = false; // BS_UI = { port: 3000 }; // false to disable UI

const SRC_ROOT      = './__build/',
      DST_ROOT      = './webroot/';

const NODE_ROOT     = './node_modules/',
      BOWER_ROOT    = './bower_components/';

const _CONFIGS      = {

    bs: {
        proxy:  BS_PROXY,
        xip:    BS_USE_XIP,
        online: BS_ONLINE,
        open:   BS_OPEN
    },

    styles: {
        watch: SRC_ROOT + 'scss/**/*.scss',
        src:   SRC_ROOT + 'scss/main.scss',
        dst:   DST_ROOT,
        file: 'style.css', // output file name
        notify: {
            title:   'styles() task completed',
            message: 'There were No errors.',
            sound: false
        }
    },

    scripts: {
        src: [
            // NODE_ROOT + '{PACKAGE1}/{JS_FILE1}.js',
            // NODE_ROOT + '{PACKAGE2}/{JS_FILE2}.js',
            // ...
            // BOWER_ROOT+ '{PACKAGE1}/{JS_FILE1}.js',
            // BOWER_ROOT+ '{PACKAGE2}/{JS_FILE2}.js',
            // ...
            SRC_ROOT + 'js/app.js'
        ],
        dst: DST_ROOT,
        file: 'scripts.js', // output file name
        notify: {
            title:   'scripts() task completed',
            message: 'There were No errors.',
            sound: false
        }
    },

    images: {
        src:   SRC_ROOT + 'assets/images/**/*.+(jpg|png|jpeg|gif|tiff)',
        dst:   DST_ROOT + 'assets/images',
        notify: {
            title: 'images() task completed',
            message: 'There were No errors.',
            sound: false
        }
    },

    php: {
        src : [
            DST_ROOT + '**/*.php' // supply more specific path
        ]
    }

};

/* task styles */
export function styles() {
    return gulp.src(_CONFIGS.styles.src)
        .pipe(size())
        .pipe(sourcemaps.init())
        .pipe(sassBi())
        .pipe(sass({

        }).on('error', sass.logError))
        .pipe(sourcemaps.write())
        .pipe(rename(_CONFIGS.styles.file))
        .pipe(gulp.dest(_CONFIGS.styles.dst))
        .pipe(size())
        .pipe(browserSync.stream())
        .pipe(gulpif(GULP_NOTIFY, notify(_CONFIGS.styles.notify)));
}

/* task scripts */
export function scripts() {
    return gulp.src(_CONFIGS.scripts.src)
        .pipe(size())
        .pipe(concat(_CONFIGS.scripts.file))
        .pipe(gulp.dest(_CONFIGS.scripts.dst))
        .pipe(size())
        .pipe(browserSync.reload({stream: true, once: true}))
        .pipe(gulpif(GULP_NOTIFY, notify(_CONFIGS.scripts.notify)));
}

/* task images */
export function images() {
    return gulp.src(_CONFIGS.images.src)
        .pipe(size())
        .pipe(imagemin({optimizationLevel: 5}))
        .pipe(gulp.dest(_CONFIGS.images.dst))
        .pipe(size())
        .pipe(gulpif(GULP_NOTIFY, notify(_CONFIGS.images.notify)));
}


// in case we need to remove intermediate files
//
// const clean = () => del([
//     DST_ROOT + 'assets/images/*',
//     DST_ROOT + _CONFIGS.styles.dst,
//     DST_ROOT + _CONFIGS.scripts.dst
// ]);
// export { clean };

/* tasks for production */
export function minifyStyles() {
    return gulp.src(_CONFIGS.styles.dst + _CONFIGS.styles.file)
        .pipe(size())
        .pipe(comebineMq())
        .pipe(cleanCss({}))
        .pipe(gulp.dest(_CONFIGS.styles.dst))
        .pipe(size());
}

export function minifyScripts() {
    var scripts = gulp.src(_CONFIGS.scripts.dst + _CONFIGS.scripts.file)
        .pipe(size())
        .pipe(uglify())
        .pipe(gulp.dest(_CONFIGS.scripts.dst))
        .pipe(size());

    // in case we have additional scripts to prepare, process them then use merge()

    return scripts; // merge(scripts, ...);
}

const production = gulp.series(
    gulp.parallel(
        minifyStyles,
        minifyScripts
    )
);
export { production };

/* default task */
export function watch() {
    browserSync.init(_CONFIGS.bs);
    gulp.watch(_CONFIGS.scripts.src,  scripts);
    gulp.watch(_CONFIGS.styles.watch, styles);
    gulp.watch(_CONFIGS.images.src,   images);
    gulp.watch(_CONFIGS.php.src).on('change', browserSync.reload);
}

export default watch;

package.json

{
  "devDependencies": {
    "babel-core": "^6.25.0",
    "babel-loader": "^7.0.0",
    "babel-preset-env": "^1.5.2",
    "babel-preset-es2015": "^6.24.1",
    "babel-register": "^6.24.1",
    "gulp-babel": "^6.1.2",
    "gulp-combine-mq": "^0.4.0",
    "gulp-sass": "^3.1.0",
    "gulp-sass-bulk-import": "^1.0.1",
    "webpack": "^2.6.1"
  },
  "dependencies": {
    "browser-sync": "^2.18.12",
    "del": "^3.0.0",
    "global": "^4.3.2",
    "globby": "^6.1.0",
    "gulp": "gulpjs/gulp.git#4.0",
    "gulp-autoprefixer": "^4.0.0",
    "gulp-changed": "^3.1.0",
    "gulp-clean-css": "^3.4.2",
    "gulp-cli": "^1.3.0",
    "gulp-combine-media-queries": "^0.2.0",
    "gulp-concat": "^2.6.1",
    "gulp-filesize": "^0.0.6",
    "gulp-filter": "^5.0.0",
    "gulp-htmlmin": "^3.0.0",
    "gulp-if": "^2.0.2",
    "gulp-imagemin": "^3.3.0",
    "gulp-load-plugins": "^1.5.0",
    "gulp-notify": "^3.0.0",
    "gulp-plumber": "^1.1.0",
    "gulp-rename": "^1.2.2",
    "gulp-ruby-sass": "^2.1.1",
    "gulp-shell": "^0.6.3",
    "gulp-sourcemaps": "^2.6.0",
    "gulp-uglify": "^3.0.0",
    "gulp-util": "^3.0.8",
    "gulp-watch": "^4.3.11",
    "merge-stream": "^1.0.1",
    "node-sass-globbing": "^0.0.23",
    "normalize-opentype.css": "^0.2.4",
    "npm": "^5.0.3",
    "object-fit": "^0.4.3",
    "parse-filepath": "^1.0.1",
    "pretty-hrtime": "^1.0.3",
    "require-dir": "^0.3.2",
    "run-sequence": "^1.2.2",
    "vinyl-source-stream": "^1.1.0",
    "watchify": "^3.9.0"
  }
}