[DAS] Fixes hover for mixin on abstract implemented type

[email protected]

Bug: #57097
Change-Id: Ia67ebb1dbca9e8b3a63e2a26dbb7234385b4bdf4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/395562
Reviewed-by: Samuel Rawlins <[email protected]>
Commit-Queue: Samuel Rawlins <[email protected]>
Reviewed-by: Brian Wilkerson <[email protected]>
Auto-Submit: Felipe Morschel <[email protected]>

Author: FMorschel

pkg/analysis_server/lib/src/computer/computer_overrides.dart

@@ -178,6 +178,11 @@ class _OverriddenElementsFinder {
     }
     // super
     _addInterfaceOverrides(class_.supertype?.element3, checkType);
+    if (class_ is MixinElement2) {
+      for (var constraint in class_.superclassConstraints) {
+        _addInterfaceOverrides(constraint.element3, true);
+      }
+    }
   }
 
   void _addSuperOverrides(

pkg/analysis_server/test/lsp/hover_test.dart

@@ -416,6 +416,23 @@ print();
     String [!a^bc!] = '';
     ''', contains('This is a string.'));
 
+  Future<void> test_method_mixin_onImplementation() async {
+    var content = '''
+abstract class A {
+  /// Documentation for [f].
+  void f();
+}
+
+abstract class B implements A {}
+
+mixin M on B {
+  @override
+  void [!f^!]() {}
+}
+''';
+    await assertStringContents(content, contains('Documentation for [f].'));
+  }
+
   Future<void> test_method_startOfParameterList() => assertStringContents('''
     class A {
       /// This is a method.