When we check the root schema during resolution, first make sure
its descendants form a tree.
Change-Id: Icc430e7df463aa84b961c28616be567f98960430
Reviewed-on: https://go-review.googlesource.com/c/tools/+/670776
LUCI-TryBot-Result: Go LUCI <golang-scoped@luci-project-accounts.iam.gserviceaccount.com>
Reviewed-by: Alan Donovan <adonovan@google.com>