Remove the dependency on `package:matcher`

[natebosch]

This package will be discontinued. We could consider adding an API based on checks, but I think it would be better to use bool Function(T) and we can (in some other place) have utilities to easily convert a Matcher or a Condition to the predicate callback.

I think most of the places that accept a Matcher today currently are typed as Object, so we can probably add support for the predicate, migrate usages that must roll atomically to use that API, then publish the breaking change that makes the signature a function type.