Firestore: in tests, fix SpecWatchFilter type to work on upcoming Typescript 2.7#310

wilhuff merged 3 commits intofirebase:masterfirebase/firebase-js-sdk:masterfrom

sandersn:firestore-fix-SpecWatchFilter-typesandersn/firebase-js-sdk:firestore-fix-SpecWatchFilter-typeCopy head branch name to clipboard