Strange unreachable error when mixing base class with other classes w/ function override. #1536

It's hard to describe the issue... so I created a repro here: https://webassembly.studio/?f=025lli7p08dc

The bug requires following conditions to be met:

1. a base class type needs to be used when creating the instance. In the example, that's line 38. If we create concrete class type, no exception.
2. we need 2 separate classes, where one inherit another. Base should have a virtual function defined, and Child should overload that. See line 11 and line 1.
3. invoke the function from a function in a class created in 1. See line 31.

Running the code should yield unreachable exception while the code above is perfectly legal.

I feel the class/interface implementation of AS isn't complete, and it's hard to debug these issues since it will usually just say "unreachable".