I'm tempted to mark this as WontFix because we can probably count the number of times we'll hit this bug with a single hand, and it's easily worked around by waiting for the second build to start.

We hit this every time we add a new builder. I'm willing to bet that'll happen more than 5 times before buildbot is gone :).

I don't think it's all that important, but it's a polish issue (for Milo), I think. I don't know why we'd even have that check.

The recipe_engine thing is unrelated, I think, and a result of clicking "force_build" on buildbot when there's no actual patch to apply in the job.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/tools/build/+/08ada733df49b7ed3ee64251b99d484c40dbcbe5

commit 08ada733df49b7ed3ee64251b99d484c40dbcbe5
Author: Edward Lemur <ehmaldonado@chromium.org>
Date: Fri Oct 06 19:09:29 2017

Fix assertion in isolate/api.py

Buildnumber can be 0:
https://build.chromium.org/p/client.webrtc.fyi/builders/WebRTC%20Perf%20Tests%20%28Win%20swarming%29/builds/0/steps/Uncaught%20Exception/logs/stdio

Bug:  753930 
Change-Id: I9ee15a88aefbab2918abb15c9b5383fb7783c5b6
Reviewed-on: https://chromium-review.googlesource.com/705377
Commit-Queue: Edward Lesmes <ehmaldonado@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>

[modify] https://crrev.com/08ada733df49b7ed3ee64251b99d484c40dbcbe5/scripts/slave/recipe_modules/isolate/api.py

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.

For more details visit https://www.chromium.org/issue-tracking/autotriage - Your friendly Sheriffbot

WontFix given comment #1 plus the fact that year has passed with no changes, we're now closer to being off buildbot.