title: rxjs audit time not working with angular 6
body: <p>I have the code like this:</p>
  
  <p>Form:</p>
  
  <pre><code>&lt;form [formGroup]="logoForm" (ngSubmit)="saveLogoInfo(logoForm)"&gt;
  
      &lt;mat-card&gt;
  
          &lt;mat-card-title&gt;Add information about your creative logo&lt;/mat-card-title&gt;
  
          &lt;mat-form-field&gt;
              &lt;input matInput placeholder="Title" #logoTitle formControlName="logoTitle" required&gt;
          &lt;/mat-form-field&gt;
  
          &lt;mat-form-field&gt;
              &lt;input matInput placeholder="Caption" formControlName="logoCaption" #logoCaption&gt;
          &lt;/mat-form-field&gt;    
  
      &lt;/mat-card&gt;
  
  &lt;/form&gt;
  </code></pre>
  
  <p>ts code:</p>
  
  <pre><code>import { Component, OnInit, ViewChild, ElementRef } from '@angular/core';
  import { LogoDesignService } from './logo-design.service';
  import { NgForm, FormGroup, FormControl, FormBuilder, Validators } from '@angular/forms';
  import { Observable } from 'rxjs/Rx';
  import { auditTime } from "rxjs/operators";
  
  
  @Component({
    selector: 'app-logo-design',
    templateUrl: './logo-design.component.html',
    styleUrls: ['./logo-design.component.css']
  })
  export class LogoDesignComponent implements OnInit {
  
    logoForm: FormGroup;
  
    @ViewChild('logoTitle') logoTitle: ElementRef;
    @ViewChild('logoCaption') logoCaption: ElementRef;
    private alive: boolean;
  
    constructor(private _logoDesignService: LogoDesignService, private fb: FormBuilder) { }
  
    ngOnInit() {
  
      this.logoForm = this.fb.group({
        logoTitle: ['', Validators.required],
        logoCaption: '',
      });
  
      this.logoForm.valueChanges.auditTime(3000).subscribe(value =&gt; {
        console.log('value', value);
      })
    }
  
    progressiveSave(logoTitle) {
      console.log('debouncing 1: ', logoTitle);
    }
  
    saveLogoInfo(form: NgForm) {
    }
  
  }
  </code></pre>
  
  <p>Package json:</p>
  
  <pre><code>"rxjs": "6.0.0",
  "rxjs-compat": "^6.2.2",
  </code></pre>
  
  <p>I get error on page: <code>ERROR TypeError: this.logoForm.valueChanges.auditTime is not a function</code></p>