* Upgrade everything except matcher.
* Roll matcher (and test)
* Adjust tests that depend on flutter:test directly to depend on a shim
* Require use of package:test shim and remove other references to package:test
This uses @kevmoo's completion package to do command line completion for flutter, and a new command "bash-completion" (with alias "zsh-completion") that will output the necessary shell script setup code, and adds the hidden command "completion" that does the actual completion.
Because it adds a dependency, I also had to do flutter update-packages --force-upgrade.
Fixes#18988.