repro.js

var u = require('uglify-es');
var code = `
(function() {
var NUMBER = '[-+]?\\d*\\.?\\d+';
var PERCENTAGE = NUMBER + '%';

function call() {
  for (var _len = arguments.length, args = Array(_len), _key = 0; _key < _len; _key++) {
    args[_key] = arguments[_key];
  }

  return '\\(\\s*(' + args.join(')\\s*,\\s*(') + ')\\s*\\)';
}

var matchers = {
  rgb: new RegExp('rgb' + call(NUMBER, NUMBER, NUMBER)),
  rgba: new RegExp('rgba' + call(NUMBER, NUMBER, NUMBER, NUMBER)),
  hsl: new RegExp('hsl' + call(NUMBER, PERCENTAGE, PERCENTAGE)),
  hsla: new RegExp('hsla' + call(NUMBER, PERCENTAGE, PERCENTAGE, NUMBER)),
  hex3: /^#([0-9a-fA-F]{1})([0-9a-fA-F]{1})([0-9a-fA-F]{1})$/,
  hex4: /^#([0-9a-fA-F]{1})([0-9a-fA-F]{1})([0-9a-fA-F]{1})([0-9a-fA-F]{1})$/,
  hex6: /^#([0-9a-fA-F]{6})$/,
  hex8: /^#([0-9a-fA-F]{8})$/
};
}());
`;

var r = u.minify(code);

console.log(r.code);