Move kernel baseline tests to front_end.

pkg/kernel/testcases/input/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;
-  Callback<T> mutableCallbackField;
-
-  Foo(this.finalField, this.callbackField);
-
-  void method(T x) {}
-
-  set setter(T x) {}
-
-  void withCallback(Callback<T> callback) {
-    callback(finalField);
-  }
-}
-
-main() {
-  Foo<int> fooInt = new Foo<int>(1, (int x) {});
-
-  fooInt.method(3);
-  fooInt.setter = 3;
-  fooInt.withCallback((int x) {});
-  fooInt.withCallback((num x) {});
-  fooInt.mutableField = 3;
-  fooInt.mutableCallbackField = (int x) {};
-
-  Foo<num> fooNum = fooInt;
-  fooNum.method(3);
-  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) {};
-}