prefer_for_elements_to_map_fromIterable reports violation only when both `key` and `value` params are present

[tijoforyou]

Describe the issue
prefer_for_elements_to_map_fromIterable reports violation only when both key and value params are present

To Reproduce

// This is linted.
var m = Map.fromIterable(iter, key: (k) => k, value: (v) => v); 

// These are not linted, but should be?
var m = Map.fromIterable(iter, key: (k) => k);
var m = Map.fromIterable(iter, value: (v) => v);

// This is also not linted, but may be okay?
var m = Map.fromIterable(iter);