Move kernel baseline tests to front_end.

pkg/front_end/test/fasta/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());
-}