I think we would need to get the max compileSdkVersion for each plugin, then check if it's grater than the project's compileSdkVersion.

I assume you mean versus warning for each plugin whose compileSdkVersion is larger?

right. I think the warning is useful, so feel free to keep it! I think providing a message in this form will be helpful:
https://github.com/flutter/flutter/blob/master/packages/flutter_tools/test/general.shard/android/gradle_errors_test.dart#L765-L774

nit: if plugin A requires 31 and plugin B requires 32, the app should bump to 32. what do you think about printing a single message at the end?

note that since the message is generated from Gradle, the error message that we would (otherwise) add to this dart file isn't necessary

This is what I was thinking:

int maxPluginCompileSdkVersion = project.android.compileSdkVersion // TODO parse as int.
int processedPlugins = getPluginList().size()
getPluginList().each { plugin ->
    Project pluginProject = project.rootProject.findProject(plugin.key)
    pluginProject.afterEvaluate {
       int pluginCompileSdkVersion = pluginProject.android.compileSdkVersion // TODO parse as int.
       maxPluginCompileSdkVersion = max(maxPluginCompileSdkVersion, pluginCompileSdkVersion)

       processedPlugins--
       if (processedPlugins == 0) {
          if (maxPluginCompileSdkVersion > project.android.compileSdkVersion) {
             // Print error message.
          }
       }
    }
}

was there an issue with the current compileSdkVersion?

Yeah, this test is looking for a specific failure, and without bumping this version, my error was given before giving the failure expected

cool, but the test was still passing, right? Your PR is mostly adding more errors to the stderr

No, the test was failing. @GaryQian can you provide some more context here?

nit: these characaters are also valid in the test. It could use Dart multiline strings if that is easier. e.g.

contains('''One or more plugins require a higher Android SDK version.

Fix this issue by adding the following to ${projectGradleFile.path}:

android {
   compileSdkVersion 31
}
''')