Fix for #992/#1004: Make sure that if the supermethod parameter is erased to Object we also use Object
AbstractTransformer #992: Make sure we don't try to find the definite type of Null

Because it's Nothing and that never helps us
AbstractTransformer #1003: Better erasure for Foo<Nothing> than Foo<?> which was invalid for method refinement
ClassTransformer: fixed a few erasure parameters
Test for #992: Bug in mixin bridge methods for default parameter values
Test for #1003: method refinement and variance edge cases
Updated test .src for #1003: erasure of Foo<Nothing>
MethodDefinitionBuilder #1004: make sure we don't try to narrow method parameters
test for #992: Bug in mixin bridge methods for default parameter values