There is a... restriction in dartdoc, such that a library which is
`@docImport`ed in one library, must be otherwise regularly imported in
some other library. Without this case, dartdoc gets a bit lost about
where the doc-imported library is.
Combine this restriction with analyzer's restriction that it doesn't
super duper handle configuration imports. Since `_goldens_io.dart` is
the "primary" import in all other files, and `_goldens_web.dart` is the
"configured" import, we must only doc-import `_goldens_io.dart`.
00d49b20e0