enable lint prefer_const_constructors_in_immutables (#12693)

* enable lint prefer_const_constructors_in_immutables

* remove LabeledGlobalKey._
diff --git a/analysis_options.yaml b/analysis_options.yaml
index 35b9e47..3c2c466 100644
--- a/analysis_options.yaml
+++ b/analysis_options.yaml
@@ -113,7 +113,7 @@
     - prefer_collection_literals
     - prefer_conditional_assignment
     - prefer_const_constructors
-    # - prefer_const_constructors_in_immutables # not yet tested
+    - prefer_const_constructors_in_immutables
     # - prefer_constructors_over_static_methods # not yet tested
     - prefer_contains
     # - prefer_expression_function_bodies # conflicts with https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo#consider-using--for-short-functions-and-methods