treat void as a top type, analyzer fix for #28763

pkg/analyzer/test/src/task/strong/checker_test.dart

 // Copyright (c) 2015, 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.
 
 library analyzer.test.src.task.strong.checker_test;
 
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 import 'strong_test_helper.dart';
 
@@ skipping 3484 matching lines @@
   void test_setterReturnTypes() {
     checkFile('''
 void voidFn() => null;
 class A {
   set a(y) => 4;
   set b(y) => voidFn();
   void set c(y) => /*error:RETURN_OF_INVALID_TYPE*/4;
   void set d(y) => voidFn();
   /*warning:NON_VOID_RETURN_FOR_SETTER*/int set e(y) => 4;
   /*warning:NON_VOID_RETURN_FOR_SETTER*/int set f(y) =>
-      /*error:RETURN_OF_INVALID_TYPE*/voidFn();
+      /*info:DOWN_CAST_IMPLICIT*/voidFn();
   set g(y) {return /*error:RETURN_OF_INVALID_TYPE*/4;}
   void set h(y) {return /*error:RETURN_OF_INVALID_TYPE*/4;}
   /*warning:NON_VOID_RETURN_FOR_SETTER*/int set i(y) {return 4;}
 }
 ''');
   }
 
   void test_setterSetterOverride() {
     checkFile('''
 class A {}
@@ skipping 624 matching lines @@
 }
     ''');
   }
 
   void test_voidSubtyping() {
     // Regression test for https://github.com/dart-lang/sdk/issues/25069
     checkFile('''
 typedef int Foo();
 void foo() {}
 void main () {
-  Foo x = /*error:INVALID_ASSIGNMENT,info:USE_OF_VOID_RESULT*/foo();
+  Foo x = /*warning:DOWN_CAST_COMPOSITE,info:USE_OF_VOID_RESULT*/foo();
 }
 ''');
   }
 }