Move kernel baseline tests to front_end.

pkg/front_end/testcases/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);
 }