Make void optional

Originally in sdk repository https://github.com/dart-lang/sdk/issues/34176

```
abstract class Executor {
O execute(I input);
}

void main() {
final exe = SomeExecutor();
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.

Start by reading the linked Dart SDK issue 34176 and the Executor example in this issue. Determine the language-design implications of allowing execute() to omit input for Executor; done means the proposal is resolved and the relevant Dart language specification behavior is defined.

Written by the indexing model from the issue text.