Check for void in rules subtyping (fixes #25069)

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.
 
 // TODO(jmesserly): this file needs to be refactored, it's a port from
 // package:dev_compiler's tests
 /// General type checking tests
 library test.src.task.strong.checker_test;
 
 import 'package:unittest/unittest.dart';
@@ skipping 258 matching lines @@
   });
 
   testChecker('Unbound type name', {
     '/main.dart': '''
       void main() {
          /*pass should be severe:StaticTypeError*/AToB y;
       }
    '''
   });
 
+  // Regression test for https://github.com/dart-lang/sdk/issues/25069
+  testChecker('Void subtyping', {
+    '/main.dart': '''
+      typedef int Foo();
+      void foo() {}
+      void main () {
+        Foo x = /*severe:StaticTypeError*/foo();
+      }
+   '''
+  });
+
   testChecker('Ground type subtyping: dynamic is top', {
     '/main.dart': '''
 
       class A {}
       class B extends A {}
 
       void main() {
          dynamic y;
          Object o;
          int i = 0;
@@ skipping 2115 matching lines @@
 
         baz1() sync* { yield* (/*info:DynamicCast*/x); }
         Iterable baz2() sync* { yield* (/*info:DynamicCast*/x); }
         Iterable<int> baz3() sync* { yield* (/*warning:DownCastComposite*/x); }
         Iterable<int> baz4() sync* { yield* new Iterable<int>(); }
         Iterable<int> baz5() sync* { yield* (/*info:InferredTypeAllocation*/new Iterable()); }
     '''
     });
   });
 }