avoid_classes_with_only_static_members with static final members

[mabdelaal86]

According to "Effective Dart: Design", it is encouraged to avoid defining a class that contains only static members, but this isn't a hard rule. For example, with constants and enum-like types, it may be natural to group them in a class.

I have an issue with static final members. I believe they should be considered as constants and it should be allowed to group static constant and final members in classes.
There are some types that don't have a constant constructor, so the only way to create constant values of such types is by using final keyword.
But the analyzer complains about such classes as they have a static final member.