def wpt_manifest(self, path):
        assert path in self.WPT_DIRS
        # Convert '/' to the platform-specific separator.
        path = self._filesystem.normpath(path)
        manifest_path = self._filesystem.join(self.web_tests_dir(), path,
                                              MANIFEST_NAME)
        if not self._filesystem.exists(manifest_path) or self.get_option(
                'manifest_update', False):
            _log.debug('Generating MANIFEST.json for %s...', path)
            WPTManifest.ensure_manifest(self, path)
        return WPTManifest(self.host, manifest_path)