The following revision refers to this bug:
  https://chromium.googlesource.com/infra/luci/luci-go.git/+/6a43384a3adb5f64ea1b71f56cb7490250144c6a

commit 6a43384a3adb5f64ea1b71f56cb7490250144c6a
Author: Marc-Antoine Ruel <maruel@chromium.org>
Date: Tue Dec 04 22:44:07 2018

[downloader] Start adding test code for isolated with tarred content

The archiver supports tarred content, but not the downloader (!)
Add a unit test to assert the broken behavior first, so that the
functionality can safely be added in a following CL.

Remove file mode from TarFile(), it's irrelevant.

Bug: 911790
Change-Id: I66d6dca1f0161c7547f6a060564cf366f6b56f95
Reviewed-on: https://chromium-review.googlesource.com/c/1361763
Reviewed-by: Robbie Iannucci <iannucci@chromium.org>
Commit-Queue: Marc-Antoine Ruel <maruel@chromium.org>

[modify] https://crrev.com/6a43384a3adb5f64ea1b71f56cb7490250144c6a/client/archiver/upload_tracker.go
[modify] https://crrev.com/6a43384a3adb5f64ea1b71f56cb7490250144c6a/client/downloader/downloader_test.go
[modify] https://crrev.com/6a43384a3adb5f64ea1b71f56cb7490250144c6a/common/isolated/isolated.go