upgrade json_serializable to 5.0.0

[SunlightBro]

#474
Mentioned in json_serializable CHANGELOG 5.0.0

• Improve names of private classes generated for toJson and fromJson.
  That's why added why I added:

String get nonPrivateConcreteName {
    return '\$${constructor.redirectedName}';
}

[SunlightBro]

I made no changes to prototypes.dart and tear_off.dart.
So I'm not quit sure why this Check format step fails:

Formatted 58 files (2 changed) in 1.76 seconds.
Formatting failed: 1
Error: Process completed with exit code 1.

[SunlightBro]

Ok I formatted prototypes.dart and tear_off.dart on the newest flutter dev channel.
I think this happened a few days ago when flutter pushed a new dev pre-release
2.4.0-0.0.pre-dev => 2.4.0-4.0.pre-dev

I also added invalid_annotation_target to the ignore for file, because I didn't see another way around the @Target annotation.

[rrousselGit]

Thanks a lot!