Actualización

app/Resources/public/assets/multiselect-two-sides/gulpfile.js

@@ -0,0 +1,19 @@
+var gulp = require('gulp');
+
+// include plug-ins
+var rename = require('gulp-rename');  
+var stripDebug = require('gulp-strip-debug');
+var uglify = require('gulp-uglify');
+
+// JS concat, strip debugging and minify
+gulp.task('scripts', function() {
+    return gulp.src(['dist/js/multiselect.js'])
+        .pipe(rename('multiselect.min.js'))
+        .pipe(stripDebug())
+        .pipe(uglify({
+            sourceMap: true
+        }))
+        .pipe(gulp.dest('dist/js/'));
+});
+
+gulp.task('default', gulp.parallel('scripts'));