Note that cast_test.dart isn't migrated yet due to #39517.
First patch set is just copying the tests over. The second patchset has
the actual meaningful changes.
Change-Id: I89233f20187b4305a776f865cde09a984423fa4f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/125920
Commit-Queue: Bob Nystrom <rnystrom@google.com>
Reviewed-by: Lasse R.H. Nielsen <lrn@google.com>
Auto-Submit: Bob Nystrom <rnystrom@google.com>