Require that main is a method that optionally takes List<String> arguments

[jacob314]

The DartVM supports cases such as

var main = () {
  return () => print('hello');
}();

but there is no advantage of writing that code instead of the simpler

main() {
  print('hello');
}

Worse, defining main as a variable instead of a method is broken in Dart2JS.
We should statically require that
main()
be a function that returns void and that only accepts an optional List parameter as arguments.

[zoechi]

That means async can't be used on main() anymore because it returns Future?

[floitschG]

I believe the request is to disallow main to be a variable containing a closure. For multiple reasons we want to support different signatures of main but we can enforce that it must be a top-level function (and not just a variable).

If that's the request, then I agree.

[munificent]

Duplicate of #23887.