Exported extensions don't show up in the docs for a library

[natebosch]

I'm working on dart-archive/stream_transform#83

When I generate docs I see all the deprecated top level methods that are exported from the "main" library, but none of the extensions. If I add an extension directly I do see it in the docs.

[keertip]

@jcollins-g , could you take a look? Thanks!

[mkustermann]

@jcollins-g This might be a blocker for D26, since all our tools need to support extension methods and the core libraries, dart:ffi in particular, are using extension methods now.

[jcollins-g]

Extensions should show up in their dartdoc canonical locations which may or may not be the same as their defined locations. Will investigate.

[jcollins-g]

Getting this correct is turning out to be more of a pain than I realized. Some discovery support is best implemented as analyzer code and I've got the beginnings of that working now.

[jcollins-g]

This has improved, but there are still significant bugs, see #2047.