Move kernel baseline tests to front_end.

pkg/front_end/test/fasta/rasta/issue_000081.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 Base {
-  int hashCode = 42;
-}
-
-class Sub extends Base {
-  int _hashCode = null;
-
-  get hashCode => _hashCode ??= super.hashCode;
-
-  foo() {
-    _hashCode ??= super.hashCode;
-  }
-}
-
-main() {
-  print(new Sub().hashCode);
-  var l = [null];
-  l[0] ??= "fisk";
-}