Attempt to avoid online lookups if CI=true

[sigurdm]

If there is a working lockfile, and the right packages are in the pub cache.

[jerel]

Just wanted to add a comment here to make a link between this issue and a pre-existing issue dart-lang/sdk#45307 as I think the solution could overlap.

We use flutter test --no-pub to avoid a package fetch on each test run but it would be nice if similar functionality could be set globally via a config file or envar. For example DART_PUB_OFFLINE=true dart test. This would allow a CI run to do a dart pub get in one place and then set the envar to block all further network access.