Inheritance doesn't match Implementors for ErrorDescription/DiagnosticsProperty (probably due to generics?)

[Hixie]

https://api.flutter.dev/flutter/foundation/ErrorDescription-class.html says it inherits from "DiagnosticsProperty", but https://api.flutter.dev/flutter/foundation/DiagnosticsProperty-class.html does not list "ErrorDescription" in its list of implementors. The lack of symmetry is confusing.

[srawlins]

Another possibility is that ErrorDescription actually extends _ErrorDiagnostic, which extends DiagnosticsProperty<List<Object>>. It's possible that the undocumented class in the middle is preventing the symmetry.

[srawlins]

Likely the same as #1623, but I'm happy to keep both open. #1623 has details about the private link.