Display all interfaces a class transitively implements

[nex3]

Right now, a class displays only the interfaces it implements directly. This means the user has to traverse through its entire inheritance tree to find everything it implements—for example, it's not clear that Utf8Decoder implements StreamTransformer, even though that's an important part of how it's used.

[sethladd]

thanks for the feedback!

[srawlins]

There is a breadcrumb trail of superclasses. [https://api.dart.dev/stable/2.9.0/dart-convert/Utf8Decoder-class.html] now shows StreamTransformerBase.