Module not found: Error: Can't resolve '../../angularfire2/firestore'

### Version info

Angular:*4.1.3*

Firebase:*4.6.0*

AngularFire:*5.0.0-rc.3*

Ionic:*3.19*

### How to reproduce these conditions

ionic cordova build **--prod**

working great without --prod.

### Debug output

**Error: ./src/app/app.module.ngfactory.js
Module not found: Error: Can't resolve '../../angularfire2/firestore'......**

This error is killing me. I tried everything.
I can not downgrade to AngularFire v4 because I need FireStore.

Start with the production path invoked by `ionic cordova build --prod` and compare it with the working non-production build. Inspect the generated `src/app/app.module.ngfactory.js` and the reported `../../angularfire2/firestore` resolution failure. Done means the production build completes without this module-not-found error.

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