Extensions on nullable class are missing from generated docs

[loic-sharma]

Summary

If an extension is on T?, dart doc will not include it on T’s API reference. Follow-up to #2021

Repro

See: https://github.com/loic-sharma/static_extension_on_nullable

Create a library with an extension methods on Widget and Widget?:

class Widget {}

extension NullableDecorators on Widget? {
  Widget padding() => Widget();
}

extension Decorators on Widget {
  Widget expanded() => Widget();
}

Expected result

dart doc includes both the .padding and .expanded extension methods on the Widget class.

Actual result

The .padding extension method is missing:

[srawlins]

Good catch!