Missing covariant check on type param

[vsmenon]

The following gives no strong mode errors and prints 'hello1' in DDC - adding a string and int without error.

Where is the missing runtime check? :-) @leafpetersen @jmesserly

typedef T Callback<T>(T val);

class Foo<T> {
  T x;

  Callback<T> _callback;

  Callback<T> get callback => _callback;
  void set callback(Callback<T> f) { _callback = f; }
}

void main() {
  Foo<int> f = new Foo<int>();
  f.callback = (x) => x + 1;

  Foo<Object> f2 = f;
  var result = f2.callback('hello');
  print(result);
}

[jmesserly]

I believe this is a dupe of #27259