readFile(path: string, callback: (err: Error, contents: Buffer) => void): void {
   const result = this._readFileSync(path);
   if (result) {
     // tslint:disable-next-line:no-any
     callback(null as any, result);
   } else {
     this._inputFileSystem.readFile(path, callback);
   }
 }

 stat(path: string, callback: (err: Error, stats: Stats) => void): void {
   const result = this._statSync(path);
   if (result) {
     // tslint:disable-next-line:no-any
     callback(null as any, result);
   } else {
     this._inputFileSystem.stat(path, callback);
   }
 }

 return new Observable(obs => {
   // TODO: remove this try+catch when issue https://github.com/ReactiveX/rxjs/issues/3740 is
   // fixed.
   try {
     const data = this.inputFileSystem.readFileSync(getSystemPath(path));
     obs.next(new Uint8Array(data).buffer as ArrayBuffer);
     obs.complete();
   } catch (e) {
     obs.error(e);
   }
 });

 readlinkSync(path: string): string {
   return this._inputFileSystem.readlinkSync(path);
 }

  readFileSync(path: string): Buffer {
    const result = this._readFileSync(path);

    return result || this._inputFileSystem.readFileSync(path);
  }

  statSync(path: string): Stats {
    const result = this._statSync(path);

    return result || this._inputFileSystem.statSync(path);
  }

 readlink(path: string, callback: (err: Error, linkString: string) => void): void {
   this._inputFileSystem.readlink(path, callback);
 }