[CP] Disallow final fields to be used in a const context in analyzer

[parlough]

Commit(s) to merge

988c301

Target

stable

Prepared changelist for beta/stable

https://dart-review.googlesource.com/c/sdk/+/339820

Issue Description

If a final field is assigned in its declaration (not in a generative constructor) in a class with a constant constructor, constant evaluation incorrectly allows it to be used as a constant. This results in the prefer_const_constructors lint incorrectly suggesting const can be added as well, with no indication that it will result in a compilation error, making it quite easy to get into this situation as it's enabled by default in flutter_lints.

class Test {
  final String fakeConst = '';
  
  const Test();
  
  List<String> te() => const [fakeConst];
                          /// ^^^^^^^^^ No error, but expect non_constant_list_element
}

What is the fix

From @kallentu:

Context: Added a field check in https://dart-review.googlesource.com/c/sdk/+/312347 which the goal back then was to cut down on more unnecessary errors, but this seemed to have caused a regression elsewhere.

Reverting this part of the change and other related tests.

Why cherry-pick

Developers can work around the issue by making some structural adjustments or ignoring the lint, but it may result in confusion from developers and issues uncaught by the analyzer. Also ignoring a lint can result in missing lints in the future.

See an example of the effects of this issue in flutter/devtools#6891.

Risk

low

Issue link(s)

#53927, flutter/devtools#6891

Extra Info

This has a test and fixes a regression in the analyzer's constant context. It doesn't affect the compilers, so issues technically would be caught by those later on or before deployment.

[athomas]

Approving based on Brian's comment on the CL.

[athomas]

Released in 3.2.3.