This finally makes the code structure seem right as well: there's just
one method (_flatten_dep), with a simpler control flow.

Also added a regression test.

Bug: 570091
Change-Id: I22ac7a3af0429a7ffd874b4b1715c0f6c72e0006
Reviewed-on: https://chromium-review.googlesource.com/608241Reviewed-by: Michael Moss <mmoss@chromium.org>
Reviewed-by: Dirk Pranke <dpranke@chromium.org>
Commit-Queue: Paweł Hajdan Jr. <phajdan.jr@chromium.org>