build/lib/tsb/transpiler.ts/TranspileWorker/next
class TranspileWorker:	next(files: Vinyl[], options: ts.TranspileOptions) {
		if (this._pending !== undefined) {
			throw new Error('BUSY');
		}
		return new Promise<Vinyl[]>((resolve, reject) => {
			this._pending = [resolve, reject, files, options, Date.now()];
			const req: TranspileReq = {
				options,
				tsSrcs: files.map(file => String(file.contents))
			};
			this._worker.postMessage(req);
		});
	}