We were lacking tests for doing a `null` lookup in const maps and sets.
Also, we were lacking tests for having type literals with user-defined
classes in const maps and sets.
Bug: https://github.com/dart-lang/sdk/issues/45908
Change-Id: I1665420ddcc71df7b47a87ee9e16d7ebbe9e13f5
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/208080
Commit-Queue: Daco Harkes <dacoharkes@google.com>
Reviewed-by: Erik Ernst <eernst@google.com>
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
