This issue was originally filed by amouravski@google.com

This is generally waiting for issue #2706 , but here's a potential solution.

typedef dynamic async0();
typedef dynamic async1(dynamic a);
typedef dynamic async2(dynamic a, dynamic b);
...

Function expectAsync(Function callback, ...) {
  if (callback is async0) {
    return expectAsync0(callback, ...);
  } else if (callback is async1) {
    return expectAsync1(callback, ...);
  } else if ( ... ) {
    ...
  } else {
    throw "Too many arguments."
  }
}

Probably not worth changing this right now, but maybe in unittest v2.