v4 Alpha - scss/_progress.scss triple colon

Created by: Schmalzy

The latest build of v4 Alpha fails to compile for me because of triple colons on a psuedo element in the following file.

scss/_progress.scss - line 34 .progress[value]::-webkit-progress-value:::before {

Looks like it was recently added on this commit. - https://github.com/twbs/bootstrap/commit/d51f0a4f816534e8c39ff58d8d4c562d2dd568fb

Is this on purpose, or is this a bug?