This change adds a rule to the SDK PRESUBMIT.py to validate that the
test matrix used by the CI/CQ builders does not contain errors.
Change-Id: Ie967e71dda76677f4db84c1e9e613d702b57069f
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/147547
Commit-Queue: Karl Klose <karlklose@google.com>
Reviewed-by: Alexander Thomas <athom@google.com>