dir function
Creates a new DirectoryDescriptor descriptor with name and contents.
Descriptor.validate requires that all descriptors in contents match
children of the physical diretory, but it doesn't require that no other
children exist. To ensure that a particular child doesn't exist, use
nothing.
Implementation
DirectoryDescriptor dir(String name, [Iterable<Descriptor>? contents]) =>
DirectoryDescriptor(name, contents ?? <Descriptor>[]);