Consider adding `sort_child_properties_last` to flutter_lints

[goderbauer]

Follows flutter convention and makes code usually more readable. Helps with tree-visualizations in IDEs. Enabled in Flutter.

[goderbauer]

We decided to recommend this and add it to the flutter lints.

@pq Does this one already have an auto-fix?

[goderbauer]

Looks like it already has an auto-fix: https://github.com/dart-lang/sdk/blob/7504ce34b03cc907f9362ce43c536c33e2f030ee/pkg/analysis_server/lib/src/services/correction/fix_internal.dart#L560. Nice!