package:dart_internal is not supported on current dev releases

[srawlins]

Its environment constraint forbids 3.1.0.

https://github.com/dart-lang/sdk/blob/main/pkg/dart_internal/pubspec.yaml

CC @natebosch @munificent @devoncarew who have bumped it before.

I don't know what the requirement is for bumping the upper limit except something like "it's getting close and we haven't removed dependencies on it."

[munificent]

I'll bump the constraint and publish a version today.