To prevent the *standalone* viewer-components from breaking, we need to ensure that the `NullL10n`-interface won't accidentally diverge from the actual `L10n`-implementations.
Currently this class contains a few "special" code-paths for the COMPONENTS build-target, which normally wouldn't be a problem. However, in this particular case that means accessing code that we don't want to include unconditionally in all builds.
This is currently implemented using build-time `require`-calls which we nowadays want to avoid, and we should strive to remove all such cases from the code-base. (Generally speaking `import` is the future, and build-tools may not always play well with a mix of both formats.)
We can easily improve things here by using sub-classing for the COMPONENTS build-target, and then use the ability to re-name when exporting (to avoid breaking existing code).
Until now we've not actually had *any* tests that ensure that the *official* PDF.js-viewer API exposes the intended functionality, which means that things can easily break accidentally.
*Please note:* This unit-test cannot (easily) be run in Node.js-environments, since the `external/webL10n/l10n.js` file contains various browser-specific functionality.
