When the user upgrades their SDK (a major or minor change, but not patch) there may be new versions of packages that were not available before (or that are required for some classes of changes). We should detect this change when we're checking if they need to run "pub get" (we can get the last version used to fetch packages from .dart_tool/package_config.json: ("generatorVersion": "2.19.0-374.0.dev")) and offer to run pub upgrade (in preference to pub get).

@devoncarew should we do this in both directions, or only one? It probably doesn't affect many users, but I realise that I often switch between SDK versions (for testing) and if I click pub upgrade on this prompt and then switch back to an older SDK, there's a possibility that I now have packages that are not valid. Would pub upgrade even fix this (I don't thinkpub downgrade is what's wanted here, because I want to go back to "the highest that is valid given this new (lower) SDK"?)