Unexpectedly formatting in new style

[goderbauer]

Given the following setup:

myproject/pubspec.yaml:

name: test
version: 0.0.1
publish_to: none

environment:
  sdk: ">=3.0.0 <4.0.0"

myproject/lib/foo.dart:

enum foo {
  entry1,
  entry2,
}

I would have expected that running dart format . in the myproject directory formats foo.dart in the old style without removing/adding any trailing commas because the lower SDK bound in the pubspec.yaml is below 3.7. However, it actually uses the new format style and removes the trailing comma:

enum foo { entry1, entry2 }

Observation: After running dart pub update the formatter actually respects the given sdk constraints and uses the old style:

enum foo {
  entry1,
  entry2,
}

This caused an actual breakage in flutter's assets-for-api-docs repository where the CI was all of the sudden (without bumping the lower SDK constraints) expecting everything to be formatted in the new style: https://github.com/flutter/assets-for-api-docs/actions/runs/11847189657/job/33016375039

Expectation: The formatter uses the correct style based on the sdk constrains in the pubspec.yaml even if dart pub update hasn't been run to avoid breaking CI this way.

I observed this behavior with the formatter from Dart SDK version 3.7.0-219.0.dev.

If it doesn't reproduce: delete the .dart_tools directory and pubpsec.lock file.

[goderbauer]

cc @Piinks, who noticed this breakage
cc @munificent

[munificent]

Oof, yeah, this is a nasty UX pitfall. I'm going to close this as a duplicate of #1597 which has a lot more context.

Sorry for the breakage. I'm still trying to figure out the best way for the formatter to help users avoid this problem without adding unbounded complexity to how it interacts with the file system.