use_string_in_part_of_directives     
            
            Use string in part of directives.
This rule is available as of Dart 2.19.0.
This rule has a quick fix available.
Details
From Effective Dart:
DO use strings in part of directives.
BAD:
part of my_library;
GOOD:
part of '../../my_library.dart';
Usage
To enable the use_string_in_part_of_directives rule,
add use_string_in_part_of_directives under linter > rules in your
analysis_options.yaml
file:
linter:
  rules:
    - use_string_in_part_of_directives