Move kernel baseline tests to front_end.

pkg/front_end/testcases/covariant_generic.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.
 typedef void Callback<T>(T x);
 
 class Foo<T> {
   final T finalField;
   final Callback<T> callbackField;
 
   T mutableField;
@@ skipping 25 matching lines @@
   fooNum.method(2.5);
   fooNum.setter = 3;
   fooNum.setter = 2.5;
   fooNum.withCallback((num x) {});
   fooNum.mutableField = 3;
   fooNum.mutableField = 2.5;
   fooNum.mutableCallbackField(3);
   fooNum.mutableCallbackField(2.5);
   fooNum.mutableCallbackField = (num x) {};
 }