This CL makes it possible to run Sky apps offline by introducing new
DartLibraryProvider subclasses, specifically one that can load directly from
the file system. A future CL will expand this functionality to work with asset
bundles as well.

Currently, the only platform that uses this functionality is Linux, which can
now load a simple Sky app without even loading a network stack. Making this
work on other platforms is work for future CLs.

TBR=eseidel@chromium.org

Review URL: https://codereview.chromium.org/1203143004.