In the legacy DAPs, our detection of which local packages are "external" (eg. from pub) depends on the paths in the pubcache:

In Dart 2.19, pub.dartlang.org changed to pub.dev which broke this check, and external package code is considered users code. This means "debug just my code" doesn't work as expected.

There are many test failures on the bots for the stable branch since 2.19 was released.