Move kernel baseline tests to front_end.

pkg/front_end/testcases/rasta/issue_000048.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 A {
   bool v1;
   num v2;
   A(bool this.v1, num this.v2);
 }
 
 class M1 {
   num v2 = 0;
 }
 
 class C = A with M1;
 
 main() {
   C c = new C(true, 2);
 }