Repro of what seems like a Meteor issue when using debugOnly packages via the new api.mainModule exporting API.

Previously we could do something like this to reference debug-only packages, which may or may not be defined:

OLD package.js
--------------
Package.describe({
  name: 'debug-only-package',
  debugOnly: true
});
...
api.export('DebugOnlyFoo');

Now we try to do this:

NEW package.js
---------------
Package.describe({
  name: 'debug-only-package',
  debugOnly: true
});
...
api.mainModule('main.js');

with main.js
----------
export const DebugOnlyFoo = 'bar';

The old way of referencing things inside debug-only packages was something like:

const DebugOnlyFooPackage = Package['debug-only-package'];

// DebugOnlyFoo may or may not be something useful...
if (DebugOnlyFooPackage) {
  ...
}

However with the new modules API:

import { DebugOnlyFoo } from 'meteor/debug-only-package';

when running in production mode (meteor --production), notice that there is an error in the JS console like:

Uncaught Error: Cannot find module 'meteor/debug-only-package'