fix a warning and add a question

@keertip and @dinhviethoa 

This fixes a warning in Workspace for an unimplemented member. Hoa, how is fullPath being used? Would it be appropriate to remove it and replace references to it with `path` or `persistToToken()`?

`path` is the full path of the resource, from the POV of the workspace. This is not related to where the resource is on disk.

`persistToToken()` is used to get an opaque token that can be used to get a reference to a Resource. So, it's used for things like persisting and restoring a Resource reference across sessions.

Alternatively, should we expose the FileEntry directly?