[firebase_core] Check `kIsWeb` in `firebase_core` (firebase#1487)

* Add kIsWeb check to defaultAppName

* Bump version of firebase_core

* Bump to 0.4.2+1 instead of 0.4.3

[firebase_auth] Add platform interface (firebase#1496)

* Move firebase_auth to firebase_auth/firebase_auth

* WIP on firebase_core_platform_interface

* Fix up tests

* Respond to first round of review comments

* Format

* Respond to 2nd round of review comments

* Rename updatePhoneNumber to updatePhoneNumberCredential

* dartfmt

* Make PlatformUser extend PlatformUserInfo

* Give _PhoneAuthCallbacks a const constructor