Move kernel baseline tests to front_end.

pkg/front_end/testcases/null_aware.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 file.
 class Foo {
   int field;
   static int staticField;
 }
 
 main() {
   Foo foo = new Foo();
   foo?.field = 5;
   Foo?.staticField = 5;
   foo.field ??= 5;
   Foo.staticField ??= 5;
   foo?.field ??= 5;
   Foo?.staticField ??= 5;
 
   int intValue = foo.field ?? 6;
   num numValue = foo.field ?? 4.5;
 }