public final class XMLInputSourceAdaptor extends Object implements Source
{@link Source} that represents an {@link XMLInputSource}.
Ideally, we should be able to have {@link XMLInputSource}
derive from {@link Source}, but the way
the {@link XMLInputSource#getSystemId()} method works is
different from the way {@link Source#getSystemId()} method works.
In a long run, we should make them consistent so that we can
get rid of this awkward adaptor class.