Firestore: mixin does not work well with persisted data for documents.

[merlinnot]

When persistence is enabled for Firestore (by calling enablePersistence method) all documents and queries are cached locally. When using Firestore(Element|Mixin) with live: false I'd expect it to set the property value to locally cached data while the fresh copy is being downloaded. This behavior should be configurable when someone does not want to rely on locally cached data despite enabling persistence.

It can be achieved by using onSnapshot instead of get method (see 2f5d602#diff-cd706bcc157b02ba8cbc5d4fb834a3eeR198)