Make void optional

[cedvdb]

Originally in sdk repository dart-lang/sdk#34176

abstract class Executor<I,O> {
   O execute(I input);
}

void main() {
  final exe = SomeExecutor<void, int>();
  final result = someExecutor.execute(null);
}

It is much clearer that the task above does not take an input, instead of a nullable input when doing someExecutor.execute(); So it'd be nice to have it optional for the times where you call execute explicitely and statically know the type.

[lrhn]

The idea is that a void-typed parameter is implicitly optional (if named or trailing positional, I assume).

This is close to #2232 (nullable means optional) in that it makes the optionality of an argument at the call-point depend on the type of the argument. That feature request makes any nullable type optional, which would include void.

[ykmnkmi]

Also #1590.