In Thread and Message we have int64_t field. In order to deprecate WebMessage and WebThread, we need to use Thread and Message directly.
int64_t is supported by Emscripten and used as BigInt - Built-in type conversions.
According to WASM_BIGINT docs, it is enabled by default, but this is false; to make it work, it needs to be added explicitly.
I verified and this change has no impact on .wasm file size.