streamDocument abstract method
Streams document data from the cloud storage at the given path
with the given documentId.
The path is a string that represents the path to the document
excluding the document id. The path must always resolve to a collection.
If the path is empty, then the document is added to "default"
collection. If the path ends in a document path, then "default" is
suffixed to the path to make it a collection path.
Path can be separated by '/' or '.'.
documentId is the id of the document. If documentId is empty, then
"default" is used as the document id.
Implementation
Stream<Map<String, dynamic>> streamDocument(String path, String documentId);