In flutter/engine#52832, I unintentionally removed the generation of missing gn confgs.

Specifically, this code used to run:

final List<BuildTarget>? selectedTargets = await targetsFromCommandLine(...);
...
// If the expected build output directory doesn't exist yet, eagerly run
// the build's GN step to try to produce it.
if (!buildDir.existsSync()) {

Notably, this still would not have addressed #148431, because the above code only handled missing directories. I'm not sure how we can detect (or if we should) broken directories, so I'll file a separate issue for discussion.