How to get all static sonst fields of a Dart class without using mirrors?

I know, this question is very similar to DART find all the fields in class. I wonder if there is any way to achieve this without using mirrors?

My use case is the following: I use the shared_preferences package to store many values. For the keys I have a class like this:

class Key {
  static const String FOO = 'foo';
  static const String BAR = 'bar';
  ...
}

I want to write a unit test that checks, that this class of constants does not contains any duplicates. The following would leed to bugs:

class Key {
  static const String FOO = 'foo';
  static const String BAR = 'bar';
  static const String ANOTHER = 'foo';
}

Since dart:mirrors does not work here, what is the best way to implement the test?