Generate valid functions with optional non-nullable arguments

[copybara-service]

Generate valid functions with optional non-nullable arguments

If a method of a mocked class returns a function with optional
yet non-nullable argument, Mockito currently produces something
like

(int x, {int y}) => /* doesn't matter */

which is not a valid Dart, since y must either be required or
have a default value. required won't fit into the desired type
and coming up with a default value is not easy, since it has to
be a constant, but dummyValue doesn't always produce one.

Fortunately, we can just widen the type here and make it nullable.