Gulp + browserify + 6to5 + source maps

Use this as your start point:

var gulp = require('gulp');
var gutil = require('gulp-util');
var sourcemaps = require('gulp-sourcemaps');
var source = require('vinyl-source-stream');
var buffer = require('vinyl-buffer');
var browserify = require('browserify');
var to5ify = require('6to5ify');
var uglify = require('gulp-uglify');

gulp.task('default', function() {
  browserify('./src/index.js', { debug: true })
    .transform(to5ify)
    .bundle()
    .on('error', gutil.log.bind(gutil, 'Browserify Error'))
    .pipe(source('bundle.js'))
    .pipe(buffer())
    .pipe(sourcemaps.init({loadMaps: true})) // loads map from browserify file
    .pipe(uglify())
    .pipe(sourcemaps.write('./')) // writes .map file
    .pipe(gulp.dest('./build'));
});