Move kernel baseline tests to front_end.

pkg/front_end/testcases/rasta/issue_000080.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.
 
 class Mixin {
   var field;
   foo() => 87;
 }
 
 class Foo extends Object with Mixin {
   foo() => super.foo();
   bar() => super.field;
 }
 
 main() {
   var f = new Foo();
   f.field = 42;
   print(f.bar());
 }