After bug 1432901 or thereabouts, only explicitly exported symbols from the module will be available to importers. We need a solution for symbols used in testing, but which we don't want to make part of the module exports.

My proposal was to agree on a convention like TEST_ONLY export symbol, and use eslint (or code in the importer) to ensure it's not used outside of tests. That would have the advantage of aligning with existing JS tooling.

Kris wants a more explicit API instead of a convention, so he proposes something like:

Cu.exportBackstage({ mockFoo, mockBar });

from inside the module, and corresponding:

const { mockFoo } = Cu.importBackstage("FooBar.jsm");

from test code.

This isn't a DOM bug.