Fix of intersection not empty

The bug caused new version of FirstTransform to compile with errors.
Fixed data race in ResolveSuper

The datarace happened because for method "transform" implemented
by ResolveSuper which disambiguated overridden methods.
Previously, there was a reference FirstTransform.this.transform
of type termRefWithSig to the method implemented in a super trait. Now the same
reference points to the newly implemented method.

Solved because ResolveSuper now generates symbolic references.
Removed println's left accidentally in code base.
Allow deep subtypes in dotc_transform.

The clause got accidentally dropped in the rebase.
Merge pull request #242 from dotty-staging/fix/mixins

Fix/mixins