use babel to write ES6 code now

gulpfile.js

var gulp = require('gulp')
var sourcemaps = require('gulp-sourcemaps')
var babel = require('gulp-babel')
var concat = require('gulp-concat')

gulp.task('default',function(){
  gulp.src('src/**/*.js')
      .pipe(sourcemaps.init())
      .pipe(concat('all.js'))
      .pipe(babel())
      .pipe(soucemaps.write('.'))
      .pipe(gulp.dest('dist'))
})