Add ignore directive for protected member linting (#330)

Author: zajrik

packages/dart_mappable_builder/lib/src/builders/mappable_builder.dart

@@ -113,6 +113,7 @@ class MappableBuilder implements Builder {
       '// GENERATED CODE - DO NOT MODIFY BY HAND\n'
       '// dart format off\n'
       '// ignore_for_file: type=lint\n'
+      '// ignore_for_file: invalid_use_of_protected_member\n'
       '// ignore_for_file: unused_element, unnecessary_cast, override_on_non_overriding_member\n'
       '// ignore_for_file: strict_raw_type, inference_failure_on_untyped_parameter\n\n'
       'part of \'$libraryPath\';\n\n'