Previously, we would show 4 progress bars, which made it very unclear
what was going on since we'd just said "downloading engine". Now each
separate thing we download has its own announcement.

Eventually we should probably only download Dart, then do the rest of
this inside the flutter tool, as necessary.

Downloading these packages from cloud storage simplifies our deployment
story because we can upload to cloud storage automatically from the
buildbot.

This patch also switches the responsibility for downloading the engine
artifacts to update_engine.sh. Centralizing this responsibility ensures
that the packages and the binaries are always in sync.