* Preserve original kernel file for isolate spawn requests.

This change gives incremental kernel files ".incremental.dill" extension thereby preserving original kernel file.

Bug: https://github.com/flutter/flutter/issues/16084
Bug: https://github.com/flutter/flutter/issues/17012
Bug: https://github.com/flutter/flutter/issues/17983

* Factor out naming logic into separate function

* Fix string vs uri