Given this class:

class Point {
  var x;
  var y;
  Point(this.x, this.y);

  operator +(Point other) {
    return new Point(x + other.x, y + other.y);
  }

  String toString() => "($x, $y)";
}

dart2js will generate:

$$.Point = {"": "Object;x>,y>",
  toString$0: function() {
    return "(" + $.S(this.x) + ", " + $.S(this.y) + ")";
  }
};
$.Point$ = function(x, y) {
  return new $.Point(x, y);
};

The latter is the constructor, and it can be generated dynamically, or even:

$.Point$ = $.Point;