You were right [the first time](https://github.com/oconnore/gaia/commit/77e49e8e275ad9746b285dabaff072445d7dc8cc#L3R3): [the actual LazyLoader executes the callback through function invocation](https://github.com/mozilla-b2g/gaia/blob/87c5eb46a31abc513b27a7f24ccfcf4f6b71295d/shared/js/lazy_loader.js#L68), so the Mock should do the same. Not that this has any effect on your tests, but we don't want future tests to rely on functionality that doesn't exist in production.