Find an elegant way to use DTDResponse in types.dart

[CoderDake]

https://dart-review.googlesource.com/c/sdk/+/342980/4/pkg/dtd/lib/src/dtd_file_system_access/types.dart

The original design said to extend the response, but it seemed to make it more cumbersome.Talk to Ben to see how we can better do this.

[kenzieschmoll]

nit: this should be tracked on the dart-lang/ repo