The Auth object that `.init()` returns may not be fully ready by the time
we start calling methods on it; however it has a `.then()` method that
gets called when it is *really* ready to go.

* Add a completer to signal when Auth2 is ready.
* Move auth2-dependent init to the `then` method.
 * onSuccess, complete the auth2 ready future.
 * onError, throw a PlatformException. This is normally triggered when
the domain:port of the web app hasn't been whitelisted on the google
sign in console, and things like that. Useful for debugging.

…s checked before calling .init()