Fix for the new hint in super_mixin_example.dart in analyzer_cli.

pkg/analyzer_cli/test/data/super_mixin_example.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.
 
 /// This produces errors normally, but --supermixin disables them.
 class Test extends Object with C {
   void foo() {}
 }
 
 abstract class B {
-  void foo();
+  void foo() {}
 }
 
 abstract class C extends B {
   void bar() {
     super.foo();
   }
 }