pub global run needs to check if it should downgrade packages

[natebosch]

If you pub global activate a package on a recent SDK, and then downgrade to an older SDK, pub global run may try to run with the same packages that may have SDK constraints in their pubspec disallowing them.

As of today you can see this with the following example:

• On a Dart 2.12.0- version. pub global activate flutter_plugin_tools. Picks up charcode version 1.2.0
• Downgrade the SDK to 2.10.5
• pub global run flutter_plugin_tools.

Precompiling executable...
Failed to precompile flutter_plugin_tools:flutter_plugin_tools:
../../.pub-cache/hosted/pub.dartlang.org/charcode-1.2.0/lib/ascii.dart:1:1: Error: The specified language version is too high. The highest supported language version is 2.10.
// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
^
../../.pub-cache/hosted/pub.dartlang.org/charcode-1.2.0/lib/charcode.dart:1:1: Error: The specified language version is too high. The highest supported language version is 2.10.
// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
^
../../.pub-cache/hosted/pub.dartlang.org/charcode-1.2.0/lib/html_entity.dart:1:1: Error: The specified language version is too high. The highest supported language version is 2.10.
// Copyright (c) 2014, the Dart project authors.  Please see the AUTHORS file
^

[mit-mit]

cc @jonasfj @sigurdm, this might be a release blocker

[jonasfj]

workaround, if the root package, in this case flutter_plugin_tools had an SDK constraint 2.12.0-0 this wouldn't have been a problem as we do test the root SDK constraint.

Hmm, I'm not sure this is new, I think the same issues existed when we shipped extension methods.
But you are more likely to hit the issue with null-safety.

There is patch in #2867

[jonasfj]

@mit-mit, please reflag if you think this is critical. From chat I think the conclusion is that this only affects users who downgrade SDK.

In either case, the fix proposed will just print a prettier error message telling the user to run: dart pub global activate <package> again.

Which is better than the error message above, but hardly the end of the world. If read probably the error message above, actually says:

The specified language version is too high. The highest supported language version is 2.10.

Which is close to indicating: that this doesn't work because you downgraded you SDK version.

[mit-mit]

I'm OK with removing critical given the new understanding.

[sigurdm]

Closing this, as I think this was fixed as a side-effect of #4410 (checking the resolution before pub run)