Move kernel baseline tests to front_end.

pkg/front_end/testcases/rasta/super_mixin.dart

 // Copyright (c) 2016, 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.md file.
 
 import "mixin_library.dart" show Mixin;
 
 class Super<S> {
   foo() => 40;
   f() => 3;
 }
 
 class C<V> extends Super<V> with Mixin<V> {
 }
 
 class D extends Super with Mixin {
 }
 
 class C2<V> = Super<V> with Mixin<V>;
 
 class D2 = Super with Mixin;
 
 main() {
   print(new C().foo());
   print(new C2().foo());
 }