Cannot invoke custom compressor option 'unused' in gulp-uglify
<p>I've passed an object to gulp-uglify to specify a compressor option, just as the docs instruct. Doesn't seem to be working.</p>

<p>I would like to set the option <code>unused: false</code> in order to process a form validation function referenced by the <code>onsubmit</code> attribute of the form element. For reasons that escape me, gulp-uglify will not honor the option.</p>

<p>Here is the task I've set:</p>

<pre><code>gulp.task('js', function (cb) {
    pump([
        gulp.src('./js/app.js'),
        gulpBrowser.browserify(),
        babel({ presets: ['es2015'] }),
        uglify({compressor: { unused: false } }).on('error', function(e) {
            console.log(e);
        }),
        gulp.dest('../public/js/')
    ]);
});
</code></pre>