Redo iteration. It was too subtle.
Write a separate recursive check for whether the schemas form a tree and
have any invalid nils.
While doing that, assign paths to schemas for better error messages,
even as we check structure. This also lets us detect re-occurring
schemas without a separate map.
Everything else, including the iterators, can now assume valid structure
and populated paths.
Change-Id: I90ec93d79d1d95c92258ad7afc94ef068fdf123f
Reviewed-on: https://go-review.googlesource.com/c/tools/+/676376
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>
Auto-Submit: Jonathan Amsterdam <jba@google.com>
b0d437f1c5