interface Stream {
  open(): void;
  dispose(): void;
}


interface File {
  open(): void;
  delete(): void;
}

function fetchResource(): Stream | File {
  return {
    open: function(){ console.log('opening'); },
    dispose: function(){ console.log('disposing'); },
    delete: function(){ console.log('delete'); }
  };
}

function isDeleteable(openable: Stream | File): openable is File {
  return (<File>openable).delete !== undefined;
}

let resource = fetchResource();
resource.open();

if (isDeleteable(resource)) {
  resource.delete();
} else {
  resource.dispose();
}