build/lib/tsb/builder.ts/LanguageServiceHost/getScriptVersion
class LanguageServiceHost:	getScriptVersion(filename: string): string {
		filename = normalize(filename);
		const result = this._snapshots[filename];
		if (result) {
			return result.getVersion();
		}
		return 'UNKNWON_FILE_' + Math.random().toString(16).slice(2);
	}