flutter_plugin_tools podspec linter catches iOS compilation errors and clang analyzer warnings, but not clang warnings. It passes in --allow-warnings because pod lib lint complains about podspec warnings in addition to "legit" clang warnings.

Example:

 -> android_alarm_manager (0.0.1)
    - WARN  | license: Missing license type.
    - WARN  | description: The description is equal to the summary.
    - WARN  | [iOS] keys: Missing primary key for `source` attribute. The acceptable ones are: `git, hg, http, svn`.
...
[!] android_alarm_manager did not pass validation, due to 3 warnings (but you can use `--allow-warnings` to ignore them).

To catch iOS platform warnings on pre-submit, we should remove --allow-warnings in the linter, but first need to clean up all the podspec warnings, even though plugins are downloaded from pub, not CocoaPods.