Potentially address @protected / package:meta

[MaryaBelanger]

package:meta seems to be under exposed, examining possible solutions to help users find it/understand it better.

Based off this issue, which points to this comment from another issue. This quote from the comment discusses areas of documentation that could be updated to help:

It actually took me a while to discover that @protected even existed, and to realize that meta was an official package, and how to configure my project to start using @protected. (I'm not using Flutter.) I still can't find any documentation that I could point someone to that tells you you need to add this to your analysis_options.yaml:

analyzer:
  errors:
    invalid_use_of_protected_member: error

Googling for invalid_use_of_protected_member does not really turn up anything. The Dart "Linter rules" page comes up as the most promising-looking hit, but the page does not contain invalid_use_of_protected_member, probably because it is not a linter rule (it's an "analyzer error"?). It's not in the Readme for the meta package, either, or the documentation on the protected annotation.

There's also nothing in the main Dart docs that would point you towards @protected on class members. The only mention of the public/private distinction is in the section on libraries.

Pulling out the potential areas to address from that quote, we have:

• dart.dev
  • There's not a ton of individual package walkthroughs on dart.dev. Adding something specific could still be an option, or more likely attaching it to some existing docs that are relevant to the topic (look at where @override or private are discussed?).
• Diagnostic messages
  • invalid_use_of_protected_member is an analyzer message, should be documented somewhere with the rest of the diagnostics? Check on that
• Readme for meta package
• Documentation on the protected annotation