The Intl.message processing avoids rewriting the user's code, it just generates a new library. As a result, in order to have it be called properly at runtime you need to pass the name and parameters so that the runtime knows them. e.g.

  foo(a, b) => Intl.message("Here's $a and $b", name:"foo", args: [a, b], description: "etc")

If we had a transformer we could rewrite these to insert the name and args automatically. We could even consider having it insert package information, so we could have different packages have different deferred libraries for their messages instead of a single global catalog.