input.coffee

prototype A
  constructor: ->
    say 1

prototype B extends A
  constructor: ->
    say 2
    super()

output.js

var A, B, _a;
A = function A() {
  return say(1);
};
B = function B() {
  say(2);
  return B.__superClass__.constructor.call(this);
};
_a = function(){};
_a.prototype = A.prototype;
B.__superClass__ = A.prototype;
B.prototype = new _a();
B.prototype.constructor = B;