code: function createWebpack$(compiler: Compiler): Observable {
    const webpack$ = Observable.create((observer) => {
      compiler.plugin('invalid', () => observer.next(null));
      compiler.plugin('failed', error => observer.error(error));
      compiler.plugin('done', stats => observer.next(stats));
    }).publishReplay(1);
    webpack$.connect();
    return webpack$;
  }
docs: Create a hot observable which emits `null` when the webpack build is in
  progress, and emits the webpack `stats` object when the build is completed.
  The most recent element is replayed to new subscribers.
  
  @param  {Compiler} compiler
  The webpack compiler
  @return {Observable}
  A hot RxJS Observable