Defuse assertion when mixing managed and unmanaged classes

[dcodeIO]

Fixes #2067 where a managed / unmanaged mismatch is diagnosed but before it is reported triggers an assertion in runtime visitor logic.

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[eXponenta]

Very strange error description.
In test case managed class try to extends unmanaged, error said that we extends a unmanaged from managed.

This means that error should be produced by this:

class Base { }

@unmanaged
class Foo extends Base {}

[dcodeIO]

The full error message is:

assemblyscript/src/diagnosticMessages.json

Line 19 in c56a7f5

"Unmanaged classes cannot extend managed classes and vice-versa.": 207,

I only used part of it for the stderr test.