The plugin tooling in script/tool in flutter/plugins would be easier to add to and maintain over time if we migrated it to NNBD. It's a non-trivial amount of code, and there isn't a pressing need, so we probably want to do it incrementally.

The source tree is extremely flat, with almost all files only importing common.dart, so once we migrate that file we'll unblock migrating any of the commands we want to.