Requires exposing this constant so that the tests can use it.

* Enable 'structNullChecks' ts compiler option.

In the majority of cases, I simply turned stuff like this:

```
function fname(param: T) {
  if (!param) throw Error("invalid param; must not be nullish");
  ...
}
```

into this:

```
function fname(param: T|null|undefined) {
  if (!param) throw Error("invalid param; must not be nullish");
  ...
}
```

A more sensible approach would be eliminate the possibility of
null/undef and eliminate the checks too, but that's a bit more involved
and this patch is already enormous. But I think these sorts of
improvements could be taken advantage of opportunistically as the code
is worked on in the future.

* Add @firebase/auth-types to dev dependencies

And also update @firebase/auth to most recent to match.

* Upgraded Typescript to 3.7.x

* Removing typeof

* Made pollCallback optional

* Using async project ID discovery API in Auth and FCM

* Fixed indentation

…728)

* Using async project ID discovery API in Auth and FCM

* Async project ID discovery mechanism for ProjectManagement and InstanceId APIs

* Update Firestore to 3.0

This is not intended to be merged, I just want to make it possible to install Firebase Admin + Firestore from a branch.

* Update package-lock.json