Fix Firestore Browser tests

[schmidt-sebastian]

This is the set of changes I needed to run yarn test:browser in packages/firestore.

Notes:

• forEachType is unused so I optimized for code size.
• I don't know how to properly depend on the newest set of Firestore types, which are needed to expose onSnapshotsInSync (cc @Feiyang1)

[Feiyang1]

No need to manually change package version. Lerna will make the new types available to all packages in the workspace in our dev environment.

Package versions are automatically updated by the release script during a release.

[mikelehen]

I agree in principle, but in practice, yarn test:browser is failing from packages/firestore. 😄 It seems like with our current setup we have a period of brokenness between when we check in a change to firestore-types and when we do the next release. Any suggestions?

[schmidt-sebastian]

I just rm -rf node_modules and re-installed. I am now getting a node_modules/@firebase/firestore-types/index.d.ts that contains onSnapshotsInSync. I will remove this change from the PR.

[mikelehen]

I don't know what the right thing to do for the package version is. I'll defer to @Feiyang1 for that. The other change LGTM.