Local changes to package referenced with `sdk:` not linkable

[gspencergoog]

Description

In PR flutter/flutter#149349 I added some new methods to the flutter_test package, and then tried to reference them from the flutter package, and Dartdoc failed when building docs for it.

I'm getting a failure in the Flutter docs build even though I had a @docImport for the flutter_test package. The link worked in the IDE, but not when building docs. I suspect that this is because dartdoc is looking at the published flutter_test package, and not the local one in the sdk, since I added a new method there in the same PR.

i'm submitting the PR without the links, and then I'll try adding the links in again and see if they work after the new methods are already present in the package.