Move kernel baseline tests to front_end.

pkg/kernel/testcases/input/override.dart

-class Foo {}
-
-class Bar extends Foo {}
-
-class Base {
-  Foo method() {
-    return new Foo();
-  }
-}
-
-class Sub extends Base {
-  Foo method() {
-    return new Bar();
-  }
-}
-
-main(List<String> args) {
-  var object = args.length == 0 ? new Base() : new Sub();
-  var a = object.method();
-  print(a);
-}