pub publish --dry-run doesn't catch some errors

[cyanglaz]

When running pub publish --dry-run, some errors are not showing up and the process is considered successful. The error that I found not showing up is:

pubspec.yaml allows Flutter SDK version prior to 1.20.0, which does not support having no `ios/` folder.
Please consider increasing the Flutter SDK requirement to ^1.20.0 or higher (environment.sdk.flutter) or create an `ios/` folder.

pub publish will catch the error and stop the publishing process.

the flutter/plugins repo uses pub publish --dry-run to determine if the package is publishable, so this is causing false positives.

Steps to reproduce

1. Create a plugin or use any existing plugin. (We might publish this plugin in the process, so it is better to use a dummy plugin, or use a dummy pub server for this)
2. Remove the ios/ folder, set the min flutter dependency in pubspec.yaml to something lower than 1.20.0, e.g:
   flutter: ">=1.12.13+hotfix.5"
   3 Run pub publish, press Y when asked, see the error above.
3. Run pub publish --dry-run, see that it is successful.

Expected behavior
The step 4 above should result an error.

[sigurdm]

The pub client cannot decide what the pub server accepts.

Some validation steps will always be server-side - these cannot be part of --dry-run.

The output of --dry-run says "The server may enforce additional checks"

For this specific check we could make a client-side validation - but I think we would actually prefer the linter to handle more of these checks (such that you would not even need --dry-run to get these).