Sometimes tests remain unapproved even though the breaking change has been reverted

[athomas]

This may be related to flakiness handling.

The change to passing was ignored on this build causing service/pause_on_unhandled_async_exceptions2_test2 to stay unapproved and keeping the build red: https://ci.chromium.org/p/dart/builders/ci.sandbox/vm-kernel-win-debug-x64/6133

[whesse]

Detect flakiness in the database as well. Show transitions to and from flakiness, maybe with UI cues that they aren't precisely pinned to the commits they are shown at.