Starting from this text:

  void f({foo}) {
  }
  main() {
    var bar;
    f(foo: bar);
  }

Position the cursor on the "bar" in the line "var bar;" and do a rename refactor to rename it to "foo".

This produces the complaint "Usage of parameter 'foo' declared in '...' will be shadowed by renamed local variable."

However, it's not a problem, since the existing usage of "foo" is the name of an optional parameter, and hence won't be shadowed.