2023 UPD: this answer may be outdated, according to some comments. Didn't have time to check

I've achived this by installing the harmony branch of grunt-contrib-uglify, which supports es6:

npm install git://github.com/gruntjs/grunt-contrib-uglify.git#harmony --save-dev

This is documented to a degree in the README for gulp-uglify. For clarity, I've slightly modified the example there to match your snippet.

You need to npm install the pump, gulp-uglify, and uglify-es packages first (along with any other packages your project needs. Then, you can setup your gulp task similar to as follows:

var uglifyjs = require('uglify-es');
var composer = require('gulp-uglify/composer');
var pump = require('pump');

var minify = composer(uglifyjs, console);

gulp.task('handleJs', function (cb) {
  var options = {};

  pump([
      gulp.src('src/frontend/xy/js/file.js'),
      minify(options),
      gulp.dest('dist')
    ],
    cb
  );
});

uglify-js only supports ES5 code as input.

uglify-es also supports ES6, but is buggy and has been abandoned.

terser is a maintained replacement for it that can also handle ES6+.