Move kernel baseline tests to front_end.

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