* Use already downloaded dart sdk for frontend_server.
This is follow-up to https://github.com/flutter/flutter/pull/14610
* Remove reference to engine dart-sdk from artifacts
* Remove unused imports
* WIP to switch back to solution where dart sdk is downloaded into bin/cache/dart-sdk, but it is 'flutter precache -a' that can download all dart sdks for other platforms under bin/cache/artifacts/engine/<platform>/dart-sdk
* Download all dart sdks when 'flutter precache -a'
Artifacts are now located in a central place.
This will enable us to downlaod artifacts when we need them (instead of
downloading them all upfront).
This also makes replacing sky_snapshot with gen_snapshot easier.