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

pkg/analyzer_plugin/test/utilities/completion/inherited_reference_contributor_test.dart

 // Copyright (c) 2017, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analyzer_plugin/protocol/protocol_common.dart';
 import 'package:analyzer_plugin/utilities/completion/completion_core.dart';
 import 'package:analyzer_plugin/utilities/completion/inherited_reference_contributor.dart';
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
@@ skipping 141 matching lines @@
     await computeSuggestions();
     expect(replacementOffset, completionOffset);
     expect(replacementLength, 0);
     assertNotSuggested('Object');
     assertNotSuggested('B');
     assertNotSuggested('a');
     assertNotSuggested('b');
     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');
     assertSuggestMethod('y2', 'A2', 'int');
   }
 
   test_method_in_class() async {
     addTestSource('''
 class A {
   void m(x, int y) {}
@@ skipping 425 matching lines @@
     assertNotSuggested('foo');
     assertNotSuggested('A');
     assertNotSuggested('x');
     assertNotSuggested('y');
     assertNotSuggested('x1');
     assertNotSuggested('y1');
     assertNotSuggested('x2');
     assertNotSuggested('y2');
   }
 }