code: function bundler(entry, options, callback) {
    make_sure(options, 'pkg')
    make_sure(options, 'cwd')
  
    if (Object(options.babel) !== options.babel && options.babel !== false) {
      options.babel = {}
    }
  
    options.babel = mix({
      presets: ['es2015']
    }, options.babel, false)
  
    return new Builder(options).parse(entry, callback)
  }
docs: ## Usage ```js builder(options) .on('warn', function () { }) .parse(filename, callback) ```