Define behavior for preprocessors.

[nathanhammond]

Things we know:

• All assets are present (lazy, eager, and imported) in assets/vendor.css.
• [BUG] CSS preprocessTree is being invoked twice.
• [BUG] pre/postprocessTree is only invoked one-level-deep.
• [BUG] The app folder isn't fully nuked inside of engines.

Things we believe:

• template and js trees contain only the root assets. (inside of pre/postprocessTree in top-level addon)

Things we don't know:

• this.import('vendor/addon.css') has all CSS in the entire hierarchy, we thought we noop'd it. Why?

[rwjblue]

All assets are present (lazy, eager, and imported) in assets/vendor.css.

I believe we got somewhat confused on this one, the only assets present in assets/vendor.css were those using app.import (this.import was handled correctly as was thigns in addon/styles of the engine or addon/styles of nested child addons). #314 fixes the app.import issue.

[BUG] CSS preprocessTree is being invoked twice.

PR has been submitted to address this ember-cli/ember-cli#6676.

[BUG] pre/postprocessTree is only invoked one-level-deep.

Has been fixed upstream in ember-cli/ember-cli#6516 and will be released in ember-cli@2.12 (first beta soon).

[BUG] The app folder isn't fully nuked inside of engines.

I'd like to separate that into a separate issue, we should likely prevent engines from having app/ folders.

this.import('vendor/addon.css') has all CSS in the entire hierarchy, we thought we noop'd it. Why?

Same as the first issue, app.import was misbehaving, and should be fixed by #314.