Move kernel baseline tests to front_end.

pkg/front_end/testcases/uninitialized_fields.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 Uninitialized {
   int x;
 }
 
 class PartiallyInitialized {
   int x;
   PartiallyInitialized(this.x);
   PartiallyInitialized.noInitializer();
 }
 
 class Initialized {
   int x;
   Initialized(this.x);
 }
 
 class Forwarding {
   int x;
   Forwarding.initialize(this.x);
   Forwarding(int arg) : this.initialize(arg);
 }
 
 int uninitializedTopLevel;
 int initializedTopLevel = 4;
 
 main() {}