Add support for core packages. (#140)
Add support for core packages. (#140)

* Add support for core packages.

Tests sometimes fail because they can't find packages from the core
images.

* Support test files ending in -0.far and .far files.

* If packages directory is passed then use pm from the same folder.
flutter/packages,flutter/packages,flutter/packages,flutter/packages,flutter/packages,flutter/packages,flutter/packages,flutter/packages,flutter/packages,flutter/packages