We use #pragma once ~350 times.

Mostly in impeller/, 316 files, but also in lib/, 9 files, web_ui/, 4 files, and shell/, 10 files (including Fuchsia, Android) and more.

Unless there is a good reason we're only sometimes using it, I'd propose to replace the remainder of the guards (604 files), and ban it's recurrence.

Or, as @stuartmorgan brings up below, the opposite. There is a check to help enforce that:

llvm-header-guard