Move kernel baseline tests to front_end.

pkg/front_end/testcases/implicit_this.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 C {
   m() {
     print("Called m");
   }
 
   testC() {
     m();
   }
 }
 
 class D extends C {
   testD() {
     m();
   }
 }
 
 main() {
   new C().testC();
   new D().testD();
 }