Interestingly, 2.3 will spit out what you want (but error on the invalid syntax).

  var Test;
  (function (Test) {
      Test[Test["a"] = "RED"] = "a";
  })(Test || (Test = {}));

While 2.4 doesn't error out, but omits the inner assignment, thus not allowing the lookup.

  var Test;
  (function (Test) {
      Test["a"] = "RED";
  })(Test || (Test = {}));