Fix ClassElement.allSupertypes to include interfaces of mixins (issue 29767)

pkg/analysis_server/test/services/completion/dart/inherited_reference_contributor_test.dart

Index: pkg/analysis_server/test/services/completion/dart/inherited_reference_contributor_test.dart
diff --git a/pkg/analysis_server/test/services/completion/dart/inherited_reference_contributor_test.dart b/pkg/analysis_server/test/services/completion/dart/inherited_reference_contributor_test.dart
index e4c1c9ad7daec8f87251fb75d9efc139a954d68f..bd52c9e09c9384c5ee552e01e8341de739bd507d 100644
--- a/pkg/analysis_server/test/services/completion/dart/inherited_reference_contributor_test.dart
+++ b/pkg/analysis_server/test/services/completion/dart/inherited_reference_contributor_test.dart
@@ -159,7 +159,7 @@ class B extends A1 with A2 {
     assertNotSuggested('foo');
     assertNotSuggested('A');
     assertSuggestField('x', 'int');
-    assertSuggestMethod('y', 'A2', 'int');
+    assertSuggestMethod('y', 'A1', 'int');
     assertSuggestField('x1', 'int');
     assertSuggestMethod('y1', 'A1', 'int');
     assertSuggestField('x2', 'int');