Possibility is to use information from the builder that compiles the test binary.

hm, I thought the conclusion is: (maybe I'm wrong, but double check)
1. If there is at least one Swarming task triggered for the step in question, use that build as valid one.
2. If no Swarming task, mark that as invalid build.
3. If no valid builds in between, bail out with clear error message.

Regarding the idea to use info from builder (Mac Builder) instead of the tester (Mac 10.9 Tests, etc), it needs more investigation and we still need to deal with new test-target (kinda ~= non-existent step).

This bug is describing the fix for this if we ever want to do anything more than setting the error and bailing out.

This issue has been Available for over a year. If it's no longer important or seems unlikely to be fixed, please consider closing it out. If it is important, please re-triage the issue.

Sorry for the inconvenience if the bug really should have been left as Available.

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This seems safe to be closing out.