Move kernel baseline tests to front_end.

pkg/kernel/testcases/input/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() {}