This is more of a blanket issue, you can look at the tests lib/js/extends_test.dart and lib/js/subtyping_test.dart for specifics on where things fail.

Some specifics on where dart2js succeeds while ddc does not:

• subtyping in regards to inheritance (type that expects a JS interop base class should accept a JS interop derived class)
• subtyping between separate JS interop types (type that expects one JS interop type should not accept another unrelated JS interop class)

I'm assuming more might pop up here.