Move kernel baseline tests to front_end.

pkg/front_end/testcases/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";
 }