fix(typeAssertions): handle class setters

See the added test case for the issue.
Fixes #894

@arv, let me know if you think there is a better way to fix this. I don't like the current solution.
I think we should rather change the SET_ACCESSOR to have the same syntax tree as regular function (ParameterList). Also, currently SET_ACCESSOR processes the function body in the "super" call and thus I need this extra hacky `_isInsideSetAccessor`.

Or maybe we can change order of the transformers, so that `TypeAssertionTransformer` is applied after the SET_ACCESSOR is transformed into a regular function?