Add an api to check if a type is exactly another type

[jakemac53]

We have the isSubtypeOf api today but there are cases where you want something more like isExactly.

The specific use case I ran into is for autoListenable. I want to be able to find the widget class for a state class. The way I want to do that is to walk up the super hierarchy until I find the State class, and then grab the type argument from that.

class MyState extends State<MyWidget> {}

Today I just ended up doing a String name check.

[scheglov]

We have something like this in the analyzer for InterfaceType. I believe CFE has something like this as well.

[jakemac53]

This is exists now