Authentication with tokens for 3rd party package repositories

[jonasfj]

Starting Dart 2.15 the command dart pub tokens add can be used to add authentication for use with 3rd party package repositories.

This is documented here: https://github.com/dart-lang/pub/blob/master/doc/repository-spec-v2.md

Feel free to ping me if you have questions, also just ping me in #hackers-pub-dev- on https://github.com/flutter/flutter/wiki/Chat

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This might break your existing authentication flow used in unpub, I don't think that was ever something we wanted to support.