Several tests need to access source files in the 'pkg' directory, and
they use various mechanisms for finding it. This CL changes
everything to use the same mechanism.
Change-Id: I146a6d72ec05a20cf07451a83eada9fb8e71a966
Reviewed-on: https://dart-review.googlesource.com/5484
Reviewed-by: Konstantin Shcheglov <scheglov@google.com>