Feature tests for instantiation to bounds, and super-bounded types

This directory was created in order to hold tests pertaining to the Dart feature instantiate to bound, which provides inference of default values for omitted type arguments. In order to handle F-bounded type parameters without introducing infinite types, this feature relies on another feature, super-bounded types, which is therefore also in focus for tests in this directory. For more details, please check the feature specifications on super-bounded types and on instantiate to bound.