Deprecate avoid_unstable_final_fields

Change-Id: I1d47dfcef9d910875833823aaf304f2945bb3254
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/340285
Reviewed-by: Phil Quitslund <[email protected]>
Commit-Queue: Brian Wilkerson <[email protected]>

Author: bwilkerson

pkg/linter/example/all.yaml

@@ -44,7 +44,6 @@ linter:
     - avoid_types_as_parameter_names
     - avoid_types_on_closure_parameters
     - avoid_unnecessary_containers
-    - avoid_unstable_final_fields
     - avoid_unused_constructor_parameters
     - avoid_void_async
     - avoid_web_libraries_in_flutter

pkg/linter/lib/src/rules/avoid_unstable_final_fields.dart

@@ -120,7 +120,8 @@ class AvoidUnstableFinalFields extends LintRule {
             name: 'avoid_unstable_final_fields',
             description: _desc,
             details: _details,
-            group: Group.errors);
+            group: Group.errors,
+            state: State.deprecated());
 
   @override
   void registerNodeProcessors(