Revert "Removed tests"

This reverts commit c06bbbef0e0a5a4637d51e22f991260537b69da1.
Setup getDeclarationFor to go to class name of referenced class if no default constructor exists.
Added tests, new mock for arbitrary dart file in empty directory and removed one-test
Merge pull request #2160 from dart-lang/umop-linking-add_instantiations

Link to class name of referenced class if no default constructor exists.