Currently we don't assume to have optional arguments before the distinguishing argument in overload resolution.

For example, assume we have
interface Foo {
  long bar(optional short arg1, Interface arg2);         // (1)
  long bar(optional short arg1, CallbackFunction arg2);
};

and we want to call

  foo.bar(interface);

in JS expecting (1) to be called, but we get an error NotEnoughArguments.