Add an --upgrade flag to the dev/update_packages.dart flag which runs
'pub upgrade' instead of 'pub get'. Tell people to use this when using
'flutter analyze' since 'pub get' doesn't have the same guarantee of
getting everything in sync.
This script runs the Flutter unit tests. By default, the script assumes you
have compiled a SkyShell in an "engine/src" that's a peer to the "flutter"
directory.
