DartTypeVisitor should be a real visitor

[stereotype441]

Currently the analyzer's DartTypeVisitor class dispatches using a bunch of is checks. We should really update it to follow the traditional visitor pattern, using an abstract accept method in DartType, which each concrete subtype overrides to call the appropriate method in the visitor class.

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/153361