[analyzer_plugin] FooMixin classes are not mixins #61768

Change-Id: Icbccc25df1f2127212adb656955b74bdbafdd323
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/460720
Reviewed-by: Kevin Moore <kevmoo@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>
Reviewed-by: Samuel Rawlins <srawlins@google.com>

pkg/analyzer_plugin/api.txt

@@ -1682,8 +1682,7 @@ package:analyzer_plugin/utilities/assist/assist.dart:
     new (constructor: DartAssistRequest Function())
     result (getter: ResolvedUnitResult)
 package:analyzer_plugin/utilities/assist/assist_contributor_mixin.dart:
-  AssistContributorMixin (class extends Object implements AssistContributor):
-    new (constructor: AssistContributorMixin Function())
+  AssistContributorMixin (mixin on AssistContributor):
     collector (getter: AssistCollector)
     addAssist (method: void Function(AssistKind, ChangeBuilder, {List<Object>? args}))
 package:analyzer_plugin/utilities/change_builder/change_builder_core.dart:

pkg/analyzer_plugin/lib/utilities/assist/assist_contributor_mixin.dart

@@ -12,7 +12,7 @@ import 'package:analyzer_plugin/utilities/change_builder/change_builder_core.dar
 ///
 /// Clients may not extend or implement this class, but are allowed to use it as
 /// a mix-in when creating a subclass of [AssistContributor].
-abstract class AssistContributorMixin implements AssistContributor {
+mixin AssistContributorMixin on AssistContributor {
   /// The collector to which assists should be added.
   AssistCollector get collector;