rails/sprockets-rails — pull request #144 by matthewd (2014-04-21 20:26:59+00:00)
Title: Ignore missing dependencies, instead of failing

Arguably, Sprockets should be capable of depending on a non-existent filename, so it knows our content would change if it appeared.

But for now, carrying on silently is the best we can do.

(per @robin850 -- https://github.com/rails/sass-rails/pull/209#issuecomment-40924999)