* 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
* Make the current command injected into the AppContext, allowing
other classes to inject the current command.
* Introduce `AssetBundleFactory`, an injected factory class for
spawning instances of `AssetBundle`. This allows other run contexts
to use custom asset bundling logic.
* Clean up RunCommand by removing a 'packages' argument that duplicated
a global argument by the same name (and for the same purpose).
Duplicate arguments are confusing and error-prone.
