watch(usePolling = false) {
        if (this.isBeingWatched) return;

        let files = this.files.get();
        let watcher = chokidar
            .watch(files, { usePolling, persistent: true })
            .on('change', this.onChange.bind(this));

        // Workaround for issue with atomic writes.
        // See https://github.com/paulmillr/chokidar/issues/591
        if (!usePolling) {
            watcher.on('raw', event => {
                if (event === 'rename') {
                    watcher.unwatch(files);
                    watcher.add(files);
                }
            });
        }

        this.isBeingWatched = true;
    }
Watch all relevant files for changes.

@param {boolean} usePolling