function InMemoryFile(fullPath, fileSystem) {
        File.call(this, fullPath, fileSystem);
    }
Represents a file that will never exist on disk - a placeholder backing file for untitled Documents. NO ONE
other than DocumentManager should create instances of InMemoryFile. It is valid to test for one (`instanceof
InMemoryFile`), but it's better to check `doc.isUntitled` where possible.

Attempts to read/write an InMemoryFile will always fail, and exists() always yields false. InMemoryFile.fullPath
is just a placeholder, and should not be displayed anywhere in the UI; fullPath IS guaranteed to be unique, however.

An InMemoryFile is not added to the filesystem index, so if you ask the filesystem anything about this
object, it won't know what you're talking about (`filesystem.getFileForPath(someInMemFile.fullPath)` will not
return someInMemFile).