title: Property 'concatMap' does not exist on type 'Observable<Object>'?
body: <p>I upgraded from Angular 4 to 5 and replaced <code>Http</code> with <code>HttpClient</code>.  I removed chained map from my <code>http.post</code> call which now returns an <code>Observable&lt;Object&gt;</code>, but in my component it is now complaining that <code>concatMap does not exist on type Observable&lt;Object&gt;</code>.  Here is an example of what I am doing:</p>
  
  <pre><code>//service
  import { HttpClient} from '@angular/common/http';
  
  constructor(private _http: HttpClient) { }
  
  registerDomain()
  {
  
  return this._http.post('/api/domain/domain/register', {});
  }
  
  //component
  registerDomain(caseId,domain) {
      return this._domainService.registerDomain(caseId,domain)
        .concatMap(operation =&gt; this.getOperationDetail(operation.OperationId,this.caseId,domain))
        .concatMap(() =&gt; this.createRecordSets(domain));
    }
  </code></pre>
  
  <p>I can see map and mergeMap on <code>Observable&lt;Object&gt;</code> but not <code>concatMap</code></p>