dartdoc is dropping type parameters from implements clause, other places

[jcollins-g]

Discovered while researching #1512: Some changes in #1509 (https://github.com/dart-lang/dartdoc/blame/master/lib/src/element_type.dart#L80) tweaked the heuristic (necessary due to dart-lang/sdk#30998) we use to determine whether to display type parameters. It appears while I improved things for generic functions, I regressed them in more common cases.

Compare return type of 'distinct' on:
https://api.dartlang.org/stable/1.24.2/dart-io/Socket-class.html
vs
https://api.dartlang.org/be/150864/dart-io/Socket-class.html

Since the new dartdoc drops the type parameter, it implies the return type's 'Stream<T>' is related to the type parameter for Socket. But it really isn't, it was explicitly specified in the 'implements' clause as List<int>.

P1 since it has high impact (implies something that is not true about the return type, with no way to tell besides looking at source code that it might be wrong).