Show "on" target in the extension member summary

[kevmoo]

see https://api.dart.dev/main/dart-js_interop_unsafe/dart-js_interop_unsafe-library.html

Including on Function or on Whatever here would be VERY helpful when scanning.

[srawlins]

Working on #2021, I think the "on" target should be in the sidebar too... hopefully it reads ok. It's just hard to make any sense of an extension method without knowing what types it is available on.