Fix enum representationsto show correct (implicit) args, const

[srawlins]

When adding support for enhanced enums, I think I overlooked some validation of "old scool" enums-with-implicit-constructors. I discovered this when trying to move all tests to use Dart Language version 3.2.0 (next PR).

Unfortunately, the analyzer's constantInitializer property for such enum values is not really meant for re-constructing what a constant's source was, and it returns code like E() for the constant initializer for E.one given enum E { one, two; }. (It should implicitly use an index of 0, and show E(0); see the de-sugaring examples in the spec.) I filed dart-lang/sdk#54988.

In this code, we also add the word const to enhanced enum values (with an explicit constructor); previously, only old-school enums had the keyword const. So now that's unified.

---

• I’ve reviewed the contributor guide and applied the relevant portions to this PR.

Contribution guidelines:

• See our contributor guide for general expectations for PRs.
• Larger or significant changes should be discussed in an issue before creating a PR.
• Contributions to our repos should follow the Dart style guide and use dart format.
• Most changes should add an entry to the changelog and may need to rev the pubspec package version.
• Changes to packages require corresponding tests.

Note that many Dart repos have a weekly cadence for reviewing PRs - please allow for some latency before initial review feedback.