Step 2: Describe your environment

• Operating System version: all
• Firebase SDK version: 4.1.0
• Firebase Product: auth

Step 3: Describe the problem

Firebase Local Emulator Suite has Authentication Emulator, but this SDK doesn't support the emulator yet.

While Realtime Database emulator support being discussed on #298, I believe emulator support can be implemented separately for each product, so I opened a new issue to discuss about Authentication Emulator support.

To support auth emulator, following implementation should be added (We can borrow the ideas of Node.js Admin SDK which already supports the emulator; cf. firebase/firebase-admin-node#1044):

1. Provide a way to configure the auth client to use the Emulator Suite.
   • To be consistent with original Node.js implementation, auth emulator support should be enabled via setting FIREBASE_AUTH_EMULATOR_HOST environment variable.
2. Emulated signer for token generation / validation.
   • According to Node.js Admin SDK implementation, emulated signer returns null string as bytes from Sign(), and returns firebase-auth-emulator@example.com from Email().
   • And also, we should add Algorithm() method to signers, to switch algorithm to sign custom tokens based on which signer the client uses (Emulated signer uses none algorithm, other signers use RS256. cf. https://github.com/firebase/firebase-admin-node/blob/dbb0c785c9b72590c19655d3ab5a39338e0cd393/src/auth/token-generator.ts#L231-L252).
3. Replace identity toolkit API endpoints with local (emulated) ones when using the emulator.

The steps above mostly been deduced from Node.js implementation, so they are incomplete perhaps.

Relevant Code:

I've submitted a PR. #414