This adds a language versioning test that shows that the language
version for a package defined in a `.packages` will have version 2.7
and whereas a package defined in a `.packages_config.json` will have
the current language version (2.8 in the test). Therefore the
former will not have the non-nullable experiment enabled even when
the package is in allowed_experiments.json.
Change-Id: Iabf5d6882e21538cfc1237d848bad03a37825dfb
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/156000
Reviewed-by: Jens Johansen <jensj@google.com>
Johnni Winther
07aa9ec332