For #61407 , we need to be able to find all widgets that of a given type. Previously I experimented with using the type name, but of course this does not handles subtypes. The actual check needs to be an is check.

Since there is no way to convert a String to a Type at runtime for use in this check, we can instead evaluate an expression which assigns a closure to a field. The idea for this was inspired by how the dart devtools adds debug functionality to older versions of flutter.

Since the reload feature is not complete yet, adds an integration test which simulates how it will eventually behave