A set of field paths on a document. Used to restrict a get or update
operation on a document to a subset of its fields. This is different from
standard field masks, as this is always scoped to a Document, and takes in
account the dynamic nature of Value.
The list of field paths in the mask. See Document.fields for a field path
read / write
The hash code for this object. [...]
A representation of the runtime type of the object.