Analysis that triggered the first try job: https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVysAELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCJ6Y2hyb21pdW0uY2hyb21pdW1vcy9saW51eC1jaHJvbWVvcy1kYmcvNzkyNS9icm93c2VyX3Rlc3RzL1UwSk9ZWFpwWjJGMGFXOXVUMkp6WlhKMlpYSkNjbTkzYzJWeVZHVnpkQzVUWlhKMlpYSlNaV1JwY21WamRBPT0MCxITTWFzdGVyRmxha2VBbmFseXNpcxgBDA

Second:
https://findit-for-me.appspot.com/waterfall/flake?key=ag9zfmZpbmRpdC1mb3ItbWVymAELEhdNYXN0ZXJGbGFrZUFuYWx5c2lzUm9vdCJiY2hyb21pdW0uY2hyb21pdW1vcy9saW51eC1jaHJvbWVvcy1kYmcvNzkyNi9icm93c2VyX3Rlc3RzL1VFUkdSWGgwWlc1emFXOXVWR1Z6ZEM1RWFYTmhZbXhsVUd4MVoybHUMCxITTWFzdGVyRmxha2VBbmFseXNpcxgBDA

Dirk, Nico: does the following error mean a bug in gn config or something? How to deal with them, just retry a build?

ninja explain: output gen/ui/file_manager/file_manager/test/gen/elements/elements_bundle.html doesn't exist
ninja explain: gen/ui/file_manager/file_manager/test.html is dirty
ninja explain: gen/ui/file_manager/file_manager/test/gen/css/text_defaults.css is dirty
ninja explain: gen/ui/file_manager/file_manager/test/gen/elements/elements_bundle.html is dirty
ninja explain: gen/ui/file_manager/file_manager/test/gen/elements/files_quick_view.html is dirty
ninja explain: gen/ui/file_manager/file_manager/test/gen/elements/files_safe_media.html is dirty
ninja explain: gen/ui/file_manager/file_manager/test/gen/elements/files_safe_media.js is dirty
ninja explain: gen/ui/file_manager/file_manager/test/gen/js/elements_importer.js is dirty
ninja explain: obj/ui/file_manager/file_manager/test/create_test_main.stamp is dirty
ninja explain: obj/ui/file_manager/file_manager/test/create_test_main.stamp is dirty

Looks like it's supposed to be written here: https://chromium.googlesource.com/chromium/src/+blame/a8d5a6706e1e653cecabe04a3c5373f270dd4257/ui/file_manager/file_manager/test/BUILD.gn

That was added 2 months ago in https://chromium.googlesource.com/chromium/src/+/e406b0dbadf2de3f754bce650332da711dce7863 ; maybe joelhockey knows what's up.

The error says that we built once, then build again, and the second build did work. This can happen if a build rule claims to write a file but then doesn't.

This is quite possibly a bug that I introduced on 9/13 with https://chromium.googlesource.com/chromium/src/+/bb4d6ee5a4b1c3f4fb3f90b3195fb659e104996b

I changed my scripts which generates output files.  It was writing some of my outputs to a different directory, and I failed to update the BUILD outputs to match.

Hopefully this should be fixed with https://chromium-review.googlesource.com/c/chromium/src/+/1240133

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/c635c06ebe25e8376972fa36ee44642e79507629

commit c635c06ebe25e8376972fa36ee44642e79507629
Author: Joel Hockey <joelhockey@chromium.org>
Date: Sun Sep 23 23:10:04 2018

CrOS FilesApp: Fix UI tests BUILD outputs

Bug:  888026 
Change-Id: Ia5524831cba189f390b650950112e86450c9bcd2
Reviewed-on: https://chromium-review.googlesource.com/1240133
Commit-Queue: Joel Hockey <joelhockey@chromium.org>
Reviewed-by: Nico Weber <thakis@chromium.org>
Cr-Commit-Position: refs/heads/master@{#593459}
[modify] https://crrev.com/c635c06ebe25e8376972fa36ee44642e79507629/ui/file_manager/file_manager/test/BUILD.gn

Many thanks Nico and Joel for the quick fix!