Flow analysis: improve debuggability of _CheckReachable in unit tests.
Flow analysis: improve debuggability of _CheckReachable in unit tests.

This improvement is helping me debug my code during the development of
field promotion.

Bug: https://github.com/dart-lang/language/issues/2020
Change-Id: I3a1a9334b7a3d094c48063cf39b4d22427ca2571
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/251281
Reviewed-by: Konstantin Shcheglov <09e4d7516628963212bf4aace2f97603d2b706e4@google.com>
Commit-Queue: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk