You can see the CL I made here: https://chromium-review.googlesource.com/c/v8/v8/+/1355144.
Note that, currently it's only supported when:
--experimental-wasm-bigintflag to v8.
A SpiderMonkey (Firefox) is also work-in-progress.
(module (func export 'fn') (result i64) (i64.const 66) ) )
(module (func (export 'fn') (param i64) (result i64) (get_local 0) ) )