title: Property 'forkJoin' does not exist on type 'typeof Observable'
body: <p>I am using the old syntax of importing forkJoin, I still get this ugly error.</p>
  
  <p>Why does it complain?</p>
  
  <pre><code> import { Injectable } from '@angular/core';
      import { HttpClient } from '@angular/common/http';
      import { Observable } from 'rxjs/Observable';
      import 'rxjs/observable/forkJoin';
      import 'rxjs/add/operator/map';
  
      @Injectable()
      export class DataService {
  
        apiUrl1: string = 'http://localhost:3000/person/?';
        apiUrl2: string = 'http://localhost:3000/facility/?';
        apiUrl3: string = 'http://localhost:3000/exposure/?';
  
        constructor(private http: HttpClient) { }
  
        getData(arg): Observable&lt;any&gt; {
          return this.http.get(this.apiUrl1 + arg.data)
              .flatMap((person: any) =&gt; Observable.forkJoin(this.http.get(this.apiUrl2 + person.val1), this.http.get(this.apiUrl3 + person.val2)))
              .map(([facility, exposure]) =&gt; exposure.val5 * facility.val3)
      }
      }
  </code></pre>