fix(afs): fix di warning

[FrozenPandaz]

Checklist

• Issue number for this PR: Can't resolve all parameters for AngularFirestore #1206 (required)
• Docs included?: yes
• Test units included?: yes
• In a clean directory, npm install, npm run build, and npm test run successfully? yes

Description

Fixing Error with Dependency Injection not being able to resolve all parameters in an Injectable class.

AngularFirestore is an Injectable class meaning its parameters must be resolvable via Dependency Injection.

Its current inability to do this, results in the warning:

Warning: Can't resolve all parameters for AngularFirestore in /XXXProjectPathXXX/node_modules/angularfire2/firestore/index.d.ts: ([object Object], ?). This will become an error in Angular v6.x

Using the injected Token allows the shouldEnablePersistence parameter to be resolvable via DI thus fixing the above warning!

Implementation Caveat:

• EnablePersistenceToken had to be moved to its own file in order to prevent circular module dependencies. A possible alternative would be to export this token from firestore.ts

Bonus:

• Slightly refactored the definition of FirestoreModule to only provide AngularFirestore as a provider
  • The EnablePersistenceToken only gets injected if it the enablePersistence function is called

Code sample

No API Changes

[davideast]

Oh my gosh, thank you so much @FrozenPandaz! This was on the top of my "stuff I need to actually do list", so I can't thank you enough.