contribute a dart fix declarative fix

[devoncarew]

Contribute a dart fix declarative fix:

• address add a dart fix declarative refactoring for the whereNotNull() => nonNulls change dart-lang/core#676
• this covers the whereNotNull() => nonNulls change (deprecating whereNotNull())
• contribute related test infra

Note that this change is currently not working; dart fix may not have seen this exact use case before (replacing a deprecated extension method with an extension getter).

cc @keertip for FYI

---

• 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.