deps/v8/src/js/string.js
function StringConcat(other /* and more */) {  // length == 1
  "use strict";
  CHECK_OBJECT_COERCIBLE(this, "String.prototype.concat");
  var s = TO_STRING(this);
  var len = arguments.length;
  for (var i = 0; i < len; ++i) {
    s = s + TO_STRING(arguments[i]);
  }
  return s;
}
function StringConcat(other /* and more */) {  // length == 1
  "use strict";
  CHECK_OBJECT_COERCIBLE(this, "String.prototype.concat");
  var s = TO_STRING(this);
  var len = arguments.length;
  for (var i = 0; i < len; ++i) {
    s = s + TO_STRING(arguments[i]);
  }
  return s;
}
ECMA-262, section 15.5.4.6