depend_on_asset now required for all assets?

[spohlenz]

(Moving this here from rails/sass-rails#203 as per @guilleiguaran's suggestion)

On a new Rails 4.1 app, there seems to have been a significant change to the way asset dependencies are handled.

Given app/assets/stylesheets/application.css.sass:

body
  background: image-url("bg.png")

this produces the following error because there is no depend_on_asset directive:

Sprockets::Rails::Helper::DependencyError: Asset depends on 'bg.png' to generate properly but has not declared the dependency

Surely, any call to the image-url function (or any of the other asset helper functions) should automatically declare this dependency?

I also found this tweet which illustrates how absurd this can potentially get: https://twitter.com/libc/status/453659390231728129

[guilleiguaran]

/cc @schneems @rafaelfranca